I've explained it about 100 times over the last 15 years, so here's 101.
First, most skills have a ratio of 0.2% per 0.1 skill (2%/1), after you hit the minimum skill for the usage. Many of these start at 0.0% chance when you first gain access to the ability, others have a starting percentage. This is known as the "gain window" for the skill. If a skill use success starts at 0%, then there is a range of 50 skill where it is possible to gain skill from that specific use. If the use starts with a success chance above 0%, then the gain window is reduced accordingly. For example, most Tailor items start at 50%, so you only have the remaining 50% (25 skill points) as the window to gain off it - UNLESS you've used a Power Scroll.
Also, in the past, some devs mentioned that some skills (or subsets of actions/systems within some skills) used a different ratio than 2/1. I think things like 2.5/1, 3/1, 3.5/1, 4/3 etc. Chivalry MIGHT have been one of these, but might also have been altered when the devs decided to break it, to allow it to be used in PvP (Chiv intensity used to use Karma like magery does Eval, and skill level was a secondary influence on success chance for things like cleanse by fire and spell durations).
Examples, for skills without a power scroll used:
- A specific blacksmith item can be created starting at 50.0 skill. at 50.0, the crafting is 0.0%; at 60.0 skill, the chance has increased to 20% (as the success chance advances 0.2% for every 0.1 skill, times 10.0 skill points). Without a power scroll being used, they can theoretically get skill gains from making that item all the way to 100.0, but gains will get few and far between after hitting 60% (80.0 skill)
- A spell becomes available to a caster at 55.0 skill. If the skill is part of the 0.2% per 0.1 branch (some of them were mentioned that the might not be) then at 70.0 skill there should be a 30% success chance. At 100.0 skill the spell will cast 90% of the time, and will be able to gain on it all the way there.
- Note that some low-level spell circles and crafting items state a 0.0 starting level, but that is because the displays for such are hard-coded to only show 0-100. In actuality, the success chance start point is a negative number, resulting in the positive chance for success at 0 skill - and therefore also reduces the skill gain window for that skill accordingly. Use from scroll (at least for mages) also lowers the effective circle by 2, for success chance (a Telekinesis from scroll has the same casting chance as a Night Sight cast normally, and a blade spirit from scroll has the same chance as a teleport cast normally - and the chance to gain is affected accordingly)
- At 70 Tailoring, a character can start making an item 50% success chance. They will hit 100% success at 95.0 skill, and therefore cannot gain on it from 95.1-100.0, unless a power scroll used.
- Some of the combat skills (Tactics, especially), use a ratio of the attacker's and defender's skill to determine their skill gain chance. As such, the skill gain window is still there, but it shifts location based on the comparative skill levels. This is why, for example, training a character or pet's Tactics is best done against something with +/-20 points of the skill level of the creature training.
And, so on. Note that crafting skills, with exceptional crafting chances, have a more complicated formula for that exceptional chance (usually starts at success chance -60, but there's an additional +0.3 per 0.1 skill from 95.1 to 100.0(artifact of the pre-PS era), so that for GM or higher skill it's "ex chance = normal chance -45")
Second: A character can gain from skill use on any attempt that has a lower success chance than their power scroll used in that skill, written as a percentage. Essentially, the skill gain window gets opened up wider (on the top end) from the application of the Power Scroll
If the skill is one that uses a 2%/1.0 ratio, then the following happens.
- If a 105 Power Scroll is used, then any ability of the skill that would normally only give gains up to 100% success, will give gains up to 105% success chance. This equates to +2.5 added to the point where the chance to gain normally goes away. Note that for things with a visible success chance (crafting skills), the display doesn't go past 100%, so you have to do the math.
- If a 110 Power Scroll is used, the skill gain chance range is extended to 110% success (+5.0 skill points beyond the unscrolled stop point, at 2/1)
- If a 115 Power Scroll is used, the skill gain chance range is extended to 115% success (+7.5 skill points beyond the unscrolled stop point, at 2/1)
- If a 120 Power Scroll is used, the skill gain chance range is extended to 120% success (+10.0 skill points beyond the unscrolled stop point, at 2/1)
Of course, if the skill uses one of the other ratios to calculate the Skill Gain Window, it gets complicated.
To go back to the earlier examples, scrolled to 120....
- A specific blacksmith item can be created starting at 50.0 skill. at 50.0, the crafting is 0.0%; at 60.0 skill, the chance has increased to 20% (as the success chance advances 0.2% for every 0.1 skill, times 10.0 skill points). Without a power scroll being used, they can theoretically get skill gains from making that item all the way to 100.0, but gains will get few and far between after hitting 60% (80.0 skill). When a 120 Power Scroll is used, the skill can now be trained (very slowly) up to 110.0 skill.
- A spell becomes available to a caster at 55.0 skill. If the skill is part of the 0.2% per 0.1 branch (some of them were mentioned that the might not be) then at 70.0 skill there should be a 30% success chance. At 100.0 skill the spell will cast 90% of the time, and will be able to gain on it all the way there. With a 120 Power Scroll, the caster would be able to gain skill from that spell up to 115.0 skill (105 was the 100% success point, with the theoretical 120% being at 115 (105+10)).
- As noted before, Scrolls for Magery (at least) lower the required skill by 2 spell circles. So, the use of Circle 8 spell scrolls would only give gains in skill as long as you'd be able to gain skill from casting level 6 spells directly.
- At 70 Tailoring, a character can start making an item 50% success chance. They will hit 100% success at 95.0 skill, and therefore cannot gain on it from 95.1-100.0, unless a power scroll used. With a 120 Power Scroll, the Tailor could gain skill making the item up to 105.0 skill. Note that this is the very principle behind nearly 20 years of crafting skill training guides on Stratics and elsewhere, as the guides were made to optimize skill gain to resource use (often crafting with a miniscule chance for a gain, until gains no longer possible, as it STILL ends up cheaper than moving to a harder items for faster gains). One of the more infamous elements of Tailor training is using Oil Cloths (1 cloth per), which can be used to gain to 99.6 without a Power Scroll, to 109.6 after using a 120 Power Scroll.
- With the skills that use a "versus" means of determining gain chance windows, this would of course only expand the window of the PS user, and not necessarily by a regular amount.
It does just take time and patience...and a subscription to Netflix~
