You don't raise the downforce at all unless it comes from changing the attitude of the truck. The only way to change the downforce is to change the aerodynamic attitude, add ballast or move weight from one location to a location further back. Adding air pressure will simply raise the back of the truck.
Pete
Pete, I think his question was more along the lines of using the airbags as a gauge of sorts. If I read it correctly, he was looking to take a static reference pressure, then compare that to the pressure during a run to see if he is making down force or not.
Your approach of adding weight while static and measuring the pressure difference at a given weight should give you a "reasonable" method of measurement. Instead of having just a 200# friend jump on, add known weights and make a chart. I don't know if the PSI increase on the bag will be linear, you can work that out by varying the weight and measuring the PSI.
I would do a couple of measurements to see, 100,150,200,250# and check the PSI at each weight level. If it is linear, you can easily calculate the amount of down force being generated by writing a simple formula and plopping in the PSI reading at the bag.
For fun, get a bathroom scale and put it on your jack. Lift the car up 50,100,150,200# and take a reading to calculate lift. Might be interesting to see if there are speeds at which down force goes away and by how much.