TRACKBAR, anyone here run one?

When setting up panhard bars for live front axles the usual is to set them such that they are slightly down towards the axle with the goal being that they are horizontal at the mid-point of the 'frequent travel range' (i.e. the normal motion range seen on pavement). This RoT is probably appropriate on a street car as well.

The Roll Center (RC) of a panhard bar system is defined as being where the center-line of the "Line of Action" (LoA; i.e. the straight line btwn the pivot points) pierces the chassis' center-line. Moving either end, or both, up or down will move that piercing-point and change the RC's elevation.

As to where you want to start with it, I'm not the guy. I'm not up enough on street car handling to even make a WAG. W/o the panhard the RC of a leaf sprung suspension is located at the height of the axle shaft center-line. Could start there and give yourself plenty of adjustment in both directions.