Measure the passenger side wheel arch to the tyre/wheel then do the same on the other side, subtract the drivers from the passengers, then half it so you know how much to cut off
Just lower your car without any panhard rod fitted. Then just measure between the panhard mounting points for the length of the new rod. Its the rod itself that shifts the axle over so without one both wheels don't shift over.
whilst stationary yes, just don't go driving round without it.
while sitting without panhard on flat level ground to further refine the position, I'd also measure distance from rim to rear arch lip (assuming they're not bashed on) and further tweak the rear axles lateral position relative to the chassis by tensioning with string or cable ties till both gaps are equal, then you measure the chassis bush - panhard bush distance and adjust the panhard length to match.