D20?! Heaven forbid!
I'm just starting to look at HARP and get a feel for it. Not sure if we will try a campaign with this system. It looks good but I think I'll stick to RMC (I like crunchy). The way that I have solved the problem with math is that I pulled most of my group from coworkers (Pharmacy techs). They know how to do math in their head quickly. So I guess as a GM you can simplify your life by going back to 1-100, but giving all your players a "Math for Dummies" book - and a Soda so that they do not take it personal ;-).
Seriously, though, why put yourself through all the extra prep? Doing double digit addition becomes quite easy with a little bit of practice and it is far more manly (D20 is for girly men).