The mods from a runic are totally random - skill doesn't play a part.
Tailoring Bonus applies to the normal chance to make, which normally goes up 0.2% for every 0.1 skill, after you get the base chance to make an item. If you don't have the base skill for the item, the talisman won't give you a chance to make it.
Tailoring Execeptional Bonus applies to the Exceptional chance only.
The Exceptional chance starts, below 95 skill, at your ability to make the item (without talisman) - 60%. So, if you have an 85% chance to craft something, you have a 25% to make it exceptional. The talisman adds to this number, and can make a negative number positive for Exceptional (example: 55% chance to craft is normally -5% exceptional. Using a +25% Exceptional talisman would make it 20%).
A leftover artifact of the system before power scrolls is that from 95.0 to 100.0, Tailor and Blacksmith gain at 0.5% exceptional per 0.1 skill, effectively giving an additional 15% to exceptional chance by GM skill, before reverting to the 0.2% per 0.1% above GM.