That is pretty low. I'm using 60.6gr of OE 1.5 in my .40-65 using Remington .45-70 brass formed to the chamber in my Shiloh. I don't find a lot of difference in powder capacity between Remington and Starline brass though. Are you using Starline .40-65, or .45-70 formed to fit?
I know of another shooter using 62gr of Swiss 1.5 in a Shiloh .40-65.
I mostly use Starline as it's way cheaper and probably more consistent than other brands to boot. The main exception is for my .45-110's where I use Norma.
Chris.