Question on Enchanting wrappings.

I use a staff with an enchantment enhancing wrapping in it. I have used a 14%, and now an 18%. Now i have begin wondering, does this do anything at all? f.example i have an enchantment working for 10 seconds, dealing damage every second. The wrappings makes this work for 11.8 seconds. But that gives no difference to a wrapping working for 11.4 seconds, as I dont get that last 12.0 damage delivered. So is the only viable wrappings either a +10% or a +20%, and everything in between is moot?

Any upgrade, not matter how small a boast is fine... long as the price you paided for the upgrade is fine, or if you got it for free then no problem

You get longer enchant lengths on some spells, maybe not with your class though, classic one would be the elementalist staple spell 60 second aura of restoration, with 14% you get 68.4 seconds, with 18% you get 70.8 seconds. I'm also not sure if the game rounds things up/down to the nearest whole number, seems that way with expertise, maybe thats the same with enchant length?