Enchantment Mods

As it stands I have an Enchanting +18% mod, and I was wondering weather the time is rounded up or the extended time is worked differently. As that is likely to make no sense to the majority I am show you want i mean:
Protective Sprit lasts 18seconds (with no weapon mod), so with the 18% mod that means the time is extended by 3.24 seconds. So in total 21.24 seconds, (I’m now assuming) its then rounded down to 21secs.
I’m deciding weather or not its worth my money buying the 20%mod instead as that which increases the time by 2.6secs. Meaning 21.6 or rounded up 22seconds.
I was enquiring to see if anyone knew how the extra enchanting actually works, because if it is rounded up as well as down to the nearest second the expense seems worth it, where as if it uses the exact actually time the purchase of a new mod seems pointless for 0.36 of a second.

I apologies if I have lost some people, I tend to ramble any productive help/links I thank you for, I have attempted searching but I got jack all up , also given guild-wiki a browse and found nout there either :P.

Zura x

Its not worth your money. 17% and above all suffice. people just pay for perfection. Its nothing to worry about.

I'm pretty sure that it's been tested and time is rounded down when it's a fraction.

yer i have a few coins, but im not quite rich enough to be flaunting my cash for boasting right just yet :P.

i thought this was likley to be the case, cheers to you for your help, now i dont even have to think about searching for a better mod.

Zura x