There are numerous recoil calculators on the web. This is my favorite because I can do comparisons....

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

NOTICE THAT THERE IS NO PLACE FOR "PRESSURE" INPUT IN RECOIL CALCULATIONS.

You need to get your head around how the "free recoil energy" relates to YOUR perception. Simply calculate the value for a combination you know to be comfortable to you.

For me, it's 1 ounce @ 1220 fps in an 8 lb gun for 300-400 rounds per day....that's about 14-15 ft-lbs for all-day comfort.

I also know that I'm perfectly OK with the above load in my 6.5 lb 20ga for a box of hunting loads in one day.... in other words, 18 ft-lbs is "tolerable" in limited amounts.

That would warn me that 1 &1/8 oz @ 1200 fps in a 6.5 lb SxS is going to hurt....22 ft-lbs.

etc., etc.