There are many variables that will factor into error residuals.
The flying height is a big one, but the size and shape of your ground targets is also just as important. The type of hardware measuring the points is an important determining factor in overall accuracy. The measured point needs to be visible for pinning the markers in the software.
Other than the flying height of 120m... What FOV and resolution do your lens and camera provide?
Also, what are your front and side overlap values?