Not sure that it works that way. My take is that if your HCG protocol is adequate, then no "backfilling" of some of these upstream hormones (pregnenolone, DHEA) would even be necessary. But it's not clear to me why one would choose 17-OH-progesterone over other upstream hormones like pregnenolone or progesterone to titrate HCG. Supplementing pregnenolone would of course muddy the picture.
Pregnenolone is not a reliable marker. Progesterone is slightly more $$. 170H-Progesterone sits atop the Cortisol cascade or minimally converts to androgens. If the test is more reliable, less costly, past the LH (hCG) conversions, and is further downstream, it makes good sense.