Can you be more specific about which manual you were looking at?
I've not used the 10 gauge, but no doubt it uses the same physics and concepts as the twelve gauge and other shotshells .....
Shot loads around 1-1/4 or more were indeed used in the 10 gauge when that shell was more popular, usually in the 19th century. But 1300-1400fps is damn fast!
If I was shooting the tenner, I'd be content with 1225-1275fps.
Contact Ballistic Products if you need data or 10ga info.