rate_to_ndx
ndx = rate_to_ndx(sn, final_rate);
ndx0 = rate_to_ndx(sn, rate0);
ndx1 = rate_to_ndx(sn, rate1);
ndx2 = rate_to_ndx(sn, rate2);
ndx3 = rate_to_ndx(sn, rate3);