I have the same gun as you and just shot my first powerbelts yesterday. My 45 seems to like 100gr of 777 ( 110 seemed just a tad too much ) pushing a 275gr areotip powerbelt out of it. I don't think I would go any higher than 110gr of 777 as its 15 percent hotter and more powerful than other blackpowders.I would start with 80 and work up although Cayugad and Triple Se7en ( Members ) could probably help out more , they have both given me invaluable information as I am pretty much a newbie to M/L.
Bottom line I was impressed with the powerbelts and their accuacy although their seems to quite a difference in kick between 85gr and 110gr of 777 or at least it was for me ! Good Luck ! Russ