Re: DIY Bowsprit
It really, really depends on what you want to fly from it, and in what conditions. The requirements for a code sail, a genniker, and a deep running asym are vastly different because of the different luff tension that is applied. So first figure out what you expect to fly from the pole, then figure out how far out it you want it to be.
If you are just flying a runner from a short pole just about anything will work. The loads are pretty light, and the shorter the pole the smaller the leaver arm. If you want to fly a code at the end of a long pole... Well the leaver arm is longer, and the halyard tension is substantial.
I wouldn't even bother trying to calculate this from first principle. Go take a look at the Selden retrofit pole, and match what you install to what they have designed. Alternatively buy it from them, I have had a lot of luck with their retrofit stuff to date.