My guess is, as long as the opto is not driven beyond the monotony region, a linear relationship between diode and phototransistor current can be assumed.
For the characteristics of this region, just two points (Id;Ic) are necessary to define it.
Further simplifying this, the first point is chosen as (0;0) and the second point just before the turnover.
And that's what you've been doing, right?
br, mf
PS. Side thought, beyond the turnover point, the loop gain decreases, which is normally beneficial for stability (OK, I have to admit, noise regulation gets worse).