You should know that BSA scopes are junk! Anybody here will tall you that. :D
Back it down to maybe 10 powder and go for it. And my guess is it's probably a x 44. The x 50 was on a earlier model scope that you don't see for sale.
I just found one in a fixed 36 X 50. Works slick on my 22 BR. My 5 other BSA scopes also work great. But remember, the BSA scopes are all junk.