66 track bar adjustment???

[degree01]

Hello, my question is this, I am going to lower my truck rear end by 4" so I was planning on buying an adjustable track bar but my question is this, how do you know how much to adjust it? Does it have to have so much play or none at all? or will you have to first adjust the rear end then place the bar? thank you in advance for your answer, I am a novice and don’t want to make a mistake that's going to cost me.

thank's