Ahh yeah that does make my idea not make much sense. I guess I'm just kind of a sucker for those "gotcha" moments.
As for the formula, I think it could also incorporate the player's stats in some way, adding them to the number they need to reach for an h-scene. This would represent the player character's personal attractiveness to the girl, as opposed to bimbo level (how open they are to sex) or their s-link level (how close you've become). You could also put a hefty negative modifier on the required number if you get either bad end, pretty much requiring you to bimboify them (or boost your stats)