I have pair of gauntlets I want to enchant with the Improved One-handed effect. The base enchantment with a Grand Soul Gem is +13%. I drink a potion with a +20% enchanting effect. Should I then be able to enchant something that has a +15% bonus to the enchantment at least? Whenever I try, it's still at 13%.

Or am I doing this wrong?

PS: I have no perks in enchanting, btw.