The gain window increases for any skill by the amount that the scroll is above 100.
So, for skills whose gain window is 50 (for example, smith - tailor is similar, but you get half the gain window for free, as you start at 50% success instead of 0%), you get an increase in the window for any item of 2.5 skill for a 105 scroll (up to 105% success chance, though it only displays up to 100%), 5.0 for a 110, 7.5 for a 115, and 10.0 for a 120 (120% success chance).
IF the skill window is 40, then the additions are 2.0/4.0/6.0/8.0 for 105/110/115/120.
Nearly all scrollable skills are either 40 or 50 windows.
The window for unraveling is 70, according the last reports. And, all the different modifiers and weights for crafting make the calculations for imbuing hard to understand, unless you just go get one of the items that TELLS you what your chance is, and alter your process to keep the item in the "Optimal" range.
Remember - the high cost imbues should be done ONCE PER INDIVIDUAL TARGET. the 18-19 successful imbues at the end should be 1 residue & 1 gem each, and done where they fall into the "Optimal" range.
You don't Imbue item "A" 20 times with high luck, then do item "B" (or items B-T) 20 times with 1% LRC. It's EACH item 1 time with the luck, THEN each item for 19 successes with the 1 LRC