Those are the only variables understood in world. However, we know for a Fact this isn't everything, since there is more control than Burn vs Flare in savants, and with hemalurgy. I see no reason why another variable couldn't be introduced to "fix" the model. However, we have to be careful we aren't pulling an epicyclical orbit style fallacy.
OK, what's the justification for setting something equal to a non-calculous function of itself?
limiting an allomancer by the investiture they can produce is obviously neccessary, but since there is a level of control in the most powerful of cases (Savants/Hemalurgy), and there are no factors that influence the push and the investiture process, I see it as being a simple matter of proportionality: the force of a push of N investiture is equivalent to the force of a push of 1 investiture multiplied by f(N). (Likely a minor degradation, since I resists I)
That makes sense.
It would model it more effectively given a static strength, which might be an incorrect assumption, but it is a reasonable assumption until we get a WoB. We need the strength of the push to decrease toward the middle and increase towards the end. And it needs a sharp increase, since it needs to be well below gravity. That sounds parabolic, though since V goes up then down, logarithmic function would look parabolic to an extent.
He pushes on it and pulls on it only while opposite it? That doesn't sound too hard, and would explain how Kel was able to aim, since he had no practice hitting curves. It's certainly how I read it the first time.