You can gain on any item whose chance to make is less than your Skill Cap that you're scrolled to. This works for both tailor and smith.
For example, if your tailor has a Tailor skill cap of 100 (no scroll used), you stop gaining from that item at 100% success.
If your are scrolled to 105, then you would gain from that item until your chance of success would be 105% (would still be displayed as 100%)
IF scrolled to 120, then you could gain on the item until you had a 120% chance to make (still displayed at 100%).
This is why you should never train crafting while wearing a talisman for that skill, unless you're crafting one of the rare items that starts at 0% instead of 40% for chance to make.
How to tell what your non-displayed chance to make is...
It differs below GM and above GM slightly, as there is a +15% chance to craft exceptional at GM+
Exceptional crafting of an item begins when you reach 60% chance to make normally (without a talisman). Both crafting and exceptional crafting advance at a 2/1 rate; so every 0.1 skill increases your chance to craft by 0.2%. As a result, every 2.5 skill = 5% of crafting skill for items you make, for measuring amount over 100.
So, at skills under GM, not scrolled, you stop gaining at 100% Normal, 40% exceptional. At GM, this would display as 100% normal, 55% exceptional.
If you have used a 105 crafter skill scroll for the skill you're training, you will stop gaining on at item at displayed (without a talisman):
45% Exceptional (if under GM), or 60% Exceptional (GM or higher skill).
This is 2.5 skill points higher than the point you became 100% to craft the item, and the equivalent of being 105% chance of success.
If scrolled to 110, this improves to
50% Exceptional (under GM skill) / 65% Exceptional (GM+ skill) / 5 skill points higher than the point you became 100% chance of success to craft it.
If scrolled to 115, this improves to
55% Exceptional (under GM skill) / 70% Exceptional (GM+ skill) / 7.5 skill points higher than the point you became 100% chance of success to craft it.
If scrolled to 120, this improves to
60% Exceptional (under GM skill) / 75% Exceptional (GM+ skill) / 10 skill points higher than the point you became 100% chance of success to craft it.
These numbers are the basis for all the "What you can craft for gains" charts for the various versions of Tailor & Smith FAQs (and the line for non-scrolled gains applies to all the scroll-less crafting skills as well).
Here's a quick chart I whipped up illustrating the above numbers... Mods, feel free to copy the chart and add it to any FAQs on Stratics.