This is probably not the kind of answer you are hoping for but I just brought the stringers together until I thought it looked good. (The boat is in another city now or I'd go and measure it for you.) I don't think where you put the bow and stern is very critical to how it will paddle for your son. The three year olds that have been in my boat have been a bit too light to stabilize it, he may need "training floats" for a while.

Ric