Useful Info v1.2
Here is some additional info to use with the Resto Loop glitch.
For those that want to make a single item with 100% spellcasting cost reduction in one or two magic schools, you need the following:
-Enchanting skill at level 100
-All 5 Enchanter perks
-The Insightful Enchanter perk
-A Grand Soul Gem
-A Fortify Enchanting potion with a strength of 276% OR a strength of 221% if you are a vampire with the Necromage perk.
If you follow these steps your item's spellcasting cost reduction will be exactly 100%. Note that 276% is the minimum needed to get this number. If the potion is 275% then the cost reduction will only be 99%.
If your Enchanting skill isn't at level 100, you don't have all the perks or are using a weaker Soul Gem the strength of the potion will naturally need to be stronger.