I built some 15m and 10m owa's a few years ago and used slightly different
tapering. What I did was to first find the resonant frequency of each
element as designed, then match the resonant frequency of the element with
the new taper by adjusting the tip length. The overall design came out very
close to the original specs, so I didn't worry about further optimization.

I used K7NV's yagi design (Yagistress) and EZNEC, but you could probably
use a variety of tools to do the same.

Tor
N4OGW