How do you do that?
Suppose you let a 20kg barbell drop, and it gets 10cm above your lowest squat position, then what is the tension at the bottom?
Add weight to the bar until it lowers to where the bar would be in the bottom of the squat.(if you can set your power rack rack spot pins here it will help out to serve as a reference mark or have someone with a tape measure measure the height of the bar while you are in the hole) The weight required to make it sag to the reference point is the weight taken off in the bottom position.