Im sure its fairly easy to check out some forums with tour divide riders who can give good pressure info for given tires, widths and weights. I dont have the experience offroad, All I know is running 2in slicks on my troll with two full front panniers and a handlebar bag I was usually using about 40-42 on paved roads , some rough, and that worked pretty good for me, at 135lbs rider weight.
40psi seems high to me for those tires, but hey, make sure your gauge is accurate and do a bunch of tests, no internet blah blah compares to you doing stuff yourself and getting a feel for what x feels like and what x-5psi etc etc ....