The threshold of pain, I measure at ~ 20 psi.
1) The most important thing in reducing pain is increasing area of the butt.
A 200 pound woman walking on you bare foot is much more comfortable than a 100 pound woman in high heels.
The high pressure per square inch caused by high heels caused Boeing to redesign their galley floors.
2) The second most important thing is recoil pad compliance.
If it is as soft as a woman's falsie, it will just compress to easily.
If it is hard as your truck tires, it will not compress.
3) The third most important thing is recoil pad thickness.
If it is too thin, there is not enough distance for deceleration.
In conclusion, we want to spread the recoil impulse over time and area to stay under 20 psi at the shoulder.
The best recoil pads I have found is the 1" thick Limbsaver grind to fit, but left unground for more area.