You would have to know your front and rear spring rates, at ride height, and the weight of your bumper.
I do happen to know Rock Krawler's diesel spring rate at ride height - 175 front and rear. That means that it would take 175Lbs to compress a spring 1 inch. This would mean that, if perfectly centered, 175lbs would compress your vehicle (4 springs) 1/4 inch or 700 lbs would compress your vehicle (4 springs) 1 inch (175 x 4).
I do not know the front factory spring rate for the diesel (I do know the back is 175.) I have been trying to track down the OEM front spring rate, but no luck so far. I am almost certain it is not as high as RK. So lets say it is 155 front 175 rear.
Quadratec lists the shipping weight of the steel bumper at 78lbs. The weight sits entirely on the front springs. Your rock rails are 100lbs split essentially evenly between front and rear springs. So your front springs are carrying 128lbs extra and your rear area carrying 50lbs extra. Your rear spring rate is 175 (times two...because you have 2 springs) = 350lbs to compress one inch. So your rear springs are compressed 1/7th of an inch (50/350) by your rock rails.
Your front springs have a (guessed) rate of 155, so 310lbs will compress your front 1 inch. You have 128lbs on them...meaning compression of .41 inches, so just under 1/2 an inch. That is assuming a 155 spring rate.
I didn't measure mine, (which came with plastic bumpers ...ugh) but installed Rugged Ridge steel bumpers on both ends with 85lb winch in the front, and Westin triple tube rock rails. Didn't notice any appreciable sag in the suspension.