turtlecreek, in my experience, the wood will dictate the spine. That is, a 23/64 fir shaft could have a lower spine than another fir shaft that is 5/16. It just depends on the wood density. You can expect similar spine groups from wood that is from the same stock, usually. So you could start with a larger diameter, say 23/64 on a couple of shafts and then adjust down if needed. With doug-fir, you could expect to drop 5-10# spine dropping from 23/64 to 11/32 and another 5-10# dropping from 11/32 to 5/16.
What wood are you working with?
Let us know how the doweler works for you. Pics would be great.