According to my math (not perfect but not to bad) 108mm's translates to 4 1/8" (someone else also posted the same a little above) 120mm translates to 4 5/8" so u need to add few ozs of lead to the nose just to get it to the rear mark. If I was you george I'd go for 3 3/4 " or about 90 - 95 mm's. Dont know how much lead that translates to but I'd say about 6oz's. Can't wait to see it fly.
James