Percentage verses Fixed Rate

Got a quick question...
I am setting up my system to support payouts for affiliates.
What I need is a place to enter a %Percentage for each sale
that is made within the site... Though what I found is a place to
set a fixed rate...

My question is, Is there a work around for this... I really need to use Percentages %

Another issue is, I have 3 paid packages total in the ProSites Plugin and the Affiliate plugin, to the best of my knowledge only supports one package.

Having a way to set pay to a percentage would drastically take care of this issue.
Thanks for your help

Right now I am using ProSites and the Affiliate Plugin