There is no one P Gain for all Tractors, everyone is different and you cannot go from 3.5 to 18. when preforming your slew test for the steering gain, ideally you would like to see Slew Time under 1000ms and an overshoot in the 5% or less range. high overshoot will show real jumpy on the line and an high ms will make it slugish on the line. but again remember every tractor is different and you can't just type in a gain and go.