the stock sub frame has a hole right beside the driver side body mount,i believe is 610 thous,just over a half inch,in the driver side first ,the pass side is semi slotted,have the sub sorta loose for this.

that will get you close enough,that's how gm did it.the other way is to measure from the lower ball joint grease fitting to the front of like the spring perchbolt area some place back there that has equal points.we use an alignment machine with lazers on the wheel area to see how far the thrust is off,been doing computerized alignments for 7-8 years now hope this helps.