Just remember,weight behind the insert has less influence than weight more forward.The further it extends back,the less.Mark the balance point of your existing,tuned arrow.When you add weight,keep your balance point the same.If you move the balance point,assuming shafts are identical,you change the tune.
You probably need app. 35 grs.I'm not sure how long an 8-32 screw that would take.Maybe you could use a shorter screw and wrap thin solder around the threaded portion.At any rate,I'd keep it as short as possible.