anesthesia endo billing

When billing for a CRNA medically supervised by an MDA, how many asa units does the MDA receive? My understanding is the CRNA gets 50% and the MDA gets less than 50% - usually 3 to 4 units in any given case that is medically supervised. I understand that endo and colonoscopies are about 7 units total - so how do you figure out the MDA portion? TX