fffg is more powerful for any volume used, so yes you would reduce the amout in a 45 cal. However, from Sabots tables, it seems in reality maybe not 10-15%. What that doesn't tell you is how much increased pressure the barrel endures, but for the most part is probably ok to use.

So with that in mind, probably dropping say 5% (5gr for easy to remember amounts) is a pretty good starting point.