Well, I cheated and used powerstrip, which puts all the values in when you enter HV clocks, though I'm still curious how to figure the above. ?
(Incidentally, even when using the forceware, the card automatically doubled the H and V - in the case of 256x240 being 29khz / 120hz. So weird that only Advancemame can access this card to do all 15khz modes.)