I actually found directions etc with some I bought a year ago, decided to read them, they had 2-3 lbs listed.
You might try looking up Taylor, a company that makes bumpers, and see if some maintenance/directions are online too.
I would agree with the firm, but not rock solid. So you will not need a lot of PSI, probably less than 10, and the 2-3 that I read is probably correct.
She drives me boat,
I drives me dinghy!