I don't think you can really compare endogenous GH to exogenous administered GH. GH and growth factors work in nanograms endogenously because they are used to signal pathways that need to be activated at certain times and places in the body. Flooding the bloodstream with GH once or twice or three times a day is very different pharmacodynamics compared to endogenously released GH.
I think you can test your IGF and then find a dose that keeps your IGF around natural levels (it would probably be quite low, depending entirely on the person). Nobody does that though because the point of supplementing with GH is to elevate your IGF beyond natural so you can get leaner, bigger, stronger, faster, whatever.
Even with all the negatives of elevated IGF we still inject GH, because IGF is one of the most anabolic substances we know of.