Pressure at the rail should be about 50-60 psi I believe. I had a problem with my pump low on power barley idled. Checked and found I only had 25 psi at the rail. Of course mine is an Xj so I don't know if the pressures will be exactly the same. But fuel injection pressures should be basically about the same across the board, give or take a few psi.
Whats your ZJ specs?
Auto zone loans out the fuel pressure test kit.
How would it jump 16psi when the engine is using fuel, vs the pump running with no fuel use?
My experience has been similar to CaliXJ - mid 40's key on, 39 running.