USCG Vessel Tonnage Calculation 1.5?

Can somebody please tell me why I sometimes read about the USCG calculating tonnage by multiplying your actual GT by 1.5. Is this true???

I was told by MPT that I qualified to sit for the 500 ton masters test. I have all the requirements less the classes they want to sell me. But I question the 360 days as master mate… on a vessel [U]over[/U] 50 tons. The thing is I have the 360 but about 260 of the days are on a vessel that is 50 tons. Will the USCG accept those days to fulfill this part of the tonnage requirements?