Best gains for tailoring and Blacksmith skills are between 60 to 80% chance of success - The devs even coded that range into the talk that George Hesphaestus gives in New Haven.
Chance to make exceptional (without talismans) is as follows:
0 to 95.0 skill = Chance to make - 60%
100.0 to 120.0 skill = chance to make - 45%
95.1 to 100.0 skill = Add 0.3% to chance to make exceptional for every 0.1 skill from 95.1 to 100.0 (where the bonus amount reaches 15%, and the exceptional gain rate slows back down to normal).
So, at 95.5%, the bonus would be (0.3% * 5 = 1.5%, which is then added into the equation for 0-95 skill; Success chance + 1.5% - 60% = Success chance - 58.5% = chance of exceptional).
Talsiman bonuses to make normally do not apply to the above equations, while Talisman exceptional bonuses add to the result, even making it possible to have a small chance of craft exceptionally when skill alone would not allow it (50% chance to craft - 60% = -10% of exceptional (displayed as 0%), that a +25% Exceptional talisman would raise to 15.0%)
Items that are 15% to 35% chance to craft exceptional are in the optimum skill gain range, above 100 Skill, though after using a 120 skill Powerscroll, you can gain (slowly) on items that are as much as 74.8% chance of exceptional, before talisman use (gains stop at 120% effective success chance, displayed as 75.0% exceptional chance when skill is 100+)