Talismen don't quite work like that, as they don't affect your skill. Rather, they affect your chance of making the item.
So for example you have a talisman of +20% success chance, +20% exceptional chance:
Say at 100 tailoring you have a 75% chance of making an item, and 20% of making the item exceptional, as shown by the item inside the menu. When you equip the talisman, these numbers will change to 95%/40% exceptional. It simply adds the numbers to the success chance/exceptional chance. It does not affect it in any other way, so it does not affect enhancing or repair chances, or by having only an +exceptional% talisman it does not increase your base chance of creating a normal item.
It is the same at 120 tailoring, so for example the same item may have 100% success chance and 60% exceptional. Equip the talisman, and this will change to 100%/80% exceptional. The talisman acts only as a straight bonus to your chance of crafting the item, not your skill level.
I hope that this is clear enough