How does the Multiband Compressor Auto-Gain work?

I’m trying to recreate the stock multiband compressor in Fab-Filter’s Pro-MB, and have about everything sounding the same apart from the auto make-up gain from Cubase’s multipressor. Is there a formula for it? (i.e, for -15 threshold at 2:1 ratio, boost gain 7.5 db per band?) I would love to know how this is working.