[Mike Taylor 3601]

I'll go through basics first, rods,pistons are weighed you find the lightest piston ,lighten others to match,how close varies, I balance +/- 1/2 grams or within 1 gram whichever way you want to say it. Weigh big ends rod find lightest,lighten rest to match,weigh small end find lightest,lighten rest to match.
Then you start figuring bobweight, reciprocating side uses piston,pin, locks, rings,small end of rod,then that weight is totaled and mutiplied by number of rods per throw ,V8 would be two, is mutiplied by recprocating factor normal 90 degree v8 use 50 % this is factor that is changed to over or underbalanced.
rotating side uses big end of rod,bearing,usaully we add 3 grams for oil then this weight is mutiplied by number of rods per throw,V8 would be 2.
then rotating and reciprocating are added together to make up what bobweights will weigh then bobweight s are installed on rod throws and crank spun on balancer,then crank is corrected for unbalance.

Are far as over or underbalance I never have done either,I know alot of blown alky,and big inch,and ultra high rpm 10K+ change percentage.
I would'nt worry about over or undrer balancing unless I was having vibration/unbalance caused problems
Hope this answer your question if not I try to answer them.
Mike Taylor 3601