Hi
if i load my 375 H&H with 300 gr around 2350-2400 fps with 62 grain fast burning powder instead of full power factory loads loaded with slow burning powders at 70 gr +.then how many % recoil decrease may be acheived? is ther anyway to calculate this?
regards
yes

Look up beartooth bullets,They have a recoil/load calculater? in the ballistics section,lots of great info on that web site....You can punch in all kinds of numbers and it will give you your foot pounds of recoil on every one..also look into the hodgon? & imr powder company data site & STEVES PAGES ,you can use a start load from there of varget and have a really good load that you can work up to full house loads if you want to pretty easy and painless...& have boo-coos of fun experimenting with the great 375H&H....
Hope this helps and good luck!

Try this out.

http://www.handloads.com/calc/recoil.asp