Considering a very small load, I'd choose 15V for VOH, given that Vin = 0V.
Anyway.
ML is an active load, it is probably biased in the saturation region (remember, for a FET saturated = BJT active). So it will act as a resistence.
Remember that the current through ML = the current through MI = I Total.
And, Vout = VDS (MI) = 15V - VDS (ML).
If MI is at cutoff (no current), then I total = 0 and VDS(ML) = 0 => Vout = 15V
Of course, as you increases Vin, the VDS(ML) is going to increase very little, but you would have to make the calculations. (Remember ID (ML) = ID (MI)).