carrier,
I don't know how much you know about control. In a PID, you have three components of your control.
(1) Proportional - the amount you correct is proportional to the error (you give the car more gas when you want it go faster, as you get close to your desired speed, you let off the pedal).
(2) Integral - the amount you correct is proportional to your cummulative error. You will likely over shoot once you use integral, but your steady state error is guarranteed to be zero if you use any integral control.
(3) Derivative - the amount you correct is proportional to the rate of change of your error.
One way to tune a PID is to set all coefficients to zero. Then increase q0 and decrease q1 proportionally until the valve moves (q0 > 0, q0=-q1) in this step. This will give you a proportional control, but won't insure a steady state error of zero. To insure a steady state error of zero, we'll add some integral control. You can do this by increasing q0 without decreasing q1. However, you may want to decrease the proportional control as you increase the integral control to make the valve react quickly AND have a steady state error of zero. Usually, in a control problem you don't need to use differential control BUT you can add differential control by decreasing q0 and q2 proportionally and increasing q1 twice as much as q0 or q2.
There is a whole field of engineering dedicated to figuring out your problem. Many ways to do it, most of them difficult.