Gary,
Every wooden bait is different, so I don't know any hard and fast formulas for how much weight to use.
When I make a balsa crank, I seal it with runny super glue, to make it waterproof temporarily, Then I use .072 sst screw eyes both as the line tie and rear hook hanger, and as a temporary belly hanger, so I can add my trebles and split rings to get a true reading of how the bait sits in the water.
Once I've got it ready, I float it in a bucket of water, to see how it sits.
I typically have a similar crank that I'm trying to match, so I'll put it in the water next to the one I'm making.
I add lead weights to the belly treble hook points, until I get the two baits to sit the same in the water.
Then I take the temp. screw eye and weights off, weigh them, including the weight of the temp. screw eye, and, if I have a belly hanger that's close, I take out the temp. screw eye, and, using the screw eye hole as a pilot, I drill a larger hole for the weighted belly hanger.
Once I've got the hole drilled and have test fit the hanger, I coat the inside of the new hole with runny super glue, and let it soak in to strengthen the balsa around the hole.
Finally, I coat the inside of the hole with gap filling super glue, and push in the belly hanger, turning it crossways, so the treble will straddle the bait and not cause too much hook rash.
Once I've got the belly weight right for that particular bait, I write it down onto the pattern I'm using, so I'll know for next time.
I hope this helps.