Pan hard bar adjusting with bags. What height to set at? Set it in the air? Noob.

[Low & Slow]

Have a set of rear bags coming in the mail today and will be setting my rear pan hard bar to center the axle. I have searched and know that the rear must be centered to ride height.

But what am I measuring to make sure the rear is centered at ride height?
From what location to where?
What happens when I air up or down from the set ride height?
Do I need to weld new brackets or will an adjustable bar simply work?
Do the measurements take place in the air or on the ground?
What do I look for that my rear axle isn't centered?

Sorry for all the questions, brand new to trucks, lowering, and airbags.