When you do the chalk test, make 3 or 4 lines next to each other with large chalk.

 

I usually run 65 or so when on trips and 50 or so when running around town unloaded...
first set of BFG's lasted 95k miles and this 2nd set of Mich are showing no wear @ 15k miles....

IMHO (and experience)
tire cupping is less an air pressure thing and more of a worn out shock thing ....

with shocks that are on their last leg, it allows minute up and down movement which 'cups' the tire wear....

good luck !

 

dang i guess i shoul dhave more then 36 lbs in my good years on my 7.3 excursion