"Cross Weight" is for full-size cars, but works just as well for RC cars. It's by sayware.com.
You need four scales (500g digitals work good). You enter the weights for each wheel and it calculates all the cross-weight percentages, gives you total weight, F/R balance, etc. Very useful -- if you have scales.
“There’s little point in writing if you can’t annoy somebody.”- Kingsley Amis