how exact do you think it needs to be?
I've heard a lot of people talk about how far out of balance their props are, but personally, I've never felt it was an issue. with a gas engine the prop could get nicked and destroy the entire engine. it would take quite a bit to even effect the majority of us, especially on smaller park fliers.
most people I know that do still fly gas and nitro use the small "hand" balances. its basically just a metal shaft you hold in your hand with rubber stops to hold the prop in place.