Actually I have read, but not verified, that the boost gauges on the 05 and 06 trucks is pretty accurate.
Stock these trucks top in at no more than 30 PSI. I was climbing hills this weekend with my 5th wheel in tow, and the most boost I saw on my Autometer Z-series was 27 PSI. The hills I was pulling were any where from 6 to 8%.
Just for reference here is the weight that I generally tow at:
Steer axle: 4100
Drive axle: 5700
Trailer Axles: 10400