First: the body is not calculated from measurements (such as weight, muscle mass or waist circumference). The measurements are approximated from the current state of the body, which is the sum result of what targets have been applied. I think in principle the distance between a set of vertex points is what is used to approximate those measurements. It is very difficult to go in the other direction.
To get an understanding of what happens when you move the weight slider, you might want to skim through this video:
https://www.youtube.com/watch?v=X13k7H3dNHoIf you want to generate bodies programmatically, I think you'd have to adopt a brute force approach and move the sliders gradually (this can be scripted) and check if the measurements end up being in the area of what you want. For different combinations of macro targets (age, gender, height...) you will most likely end up moving the weight slider to different positions to acheive a +-30% change.