As the first approximation you should go with the number listed on the truck, not the max for the tire.
If you wanna get scientific about it (which isn't all that hard actually), look up the tire's inflation vs. load table. You don't need a specific one for that particular tire. But you should be able to find a table for a tire with that load and speed index. Here is a chart for my tires for example:
Find corresponding tire load capacity at specified tire inflation pressures for LT315/75R16 tires to maintain proper tire inflation for a safe drive.
tirepressure.com
from there, you take the truck's spec for the max load on the front and rear axles, divide each by 2 (2 tires per axle, right? unless you run 4 in the back), and then you see what pressure is required to support that kind of load. That would be the max pressure you use, unless you overload your truck regularly.
In my case, I run mine at 38psi front and rear. The 4 tires, at that pressure, would support close to 11,000 lbs. I get reasonably "good" mileage (8-11 mpg) and a relatively smooth ride. I could dial back a little, but sometimes I load that thing to its limits and I don't want to constantly run my little compressor on these relatively large tires.