Hi: this is a questuion that goes back to the early '70's and the advent of hydraulic backstay adjusters. All of a sudden we had all the tension we could ever dream of having. How much is enough.
The key to understand this is to know that your dial reads in PSI and not actual pounds of tension. I suspect you have to multiply your dial by a factor based upon the dimensions of your cylinder to get the actual tension, 2.79 is a number that remains in my memory. But 3.14, 38D, 1.69 and 789-6400 are other numbers that remain in my memory.
So, here's what you do. You cover the dial with duct tape and
pump up the backstay till things look right and you call it good. Figure on most 40'ers the backstay will be at least a 5/16" dia wide so that's good for 12,500 lbs. and you are not going to get there.