Roegs, I set up by pressure, with 25 lbs in bags unloaded, which will reach about 40lbs or so when I set the 5-er on it. This seems to make the ride smoooooth, (also carry 75 lbs. in rear tires) and keeps the truck at a positive rake. I hate a squattin' truck, it looks like a dog scootin' across the lawn. When empty, I keep 5-10 lbs. It seems to raise an empty truck about 3/4 inch by my eye. I carry a compressor when towing, but do not have it hooked into the bags, so I fill em and keep em there.
Now, how does that relate to a tow behind?? I would experiment, load er up and air the bags so as not to raise excessively, and see how she rides. I am guessin' that in the 15 lbs range might be close.