I've seen the third order Taylor series used, but with the coefficients calculated at various offsets for a quarter wave. So you lookuo where you are in the qua

I've seen the third order Taylor series used, but with the coefficients calculated at various offsets for a quarter wave. So you lookuo where you are in the quarter wave, then look up the 3 or 4 cofficients. This keeps the error somewhat bounded as the size of X is a small so the series does not diverge too much.