What do you have? Rear sub-frame? Housing? 4 link bars? Shock kit?

I would decide what diameter tire will be used. That will tell you how high from ground level the axle will be and therefore how high the holes in the axle bracket will be. That information will tell you how high from ground level the chassis bracket and therefore the sub-frame should be. With only 3 holes for the lower bar in the chassis bracket, location will be important to having usable adjustments.
I would decide how low (or high) you will want to have the car at ride height. You don't want to sit the car on the ground and have the 4 link way out and have no way of getting it correct.

The sub-frame (4 link) needs to be a specific distance off the ground depending on what the tire diameter is to keep the axle/chassis bracket relationship correct. The desired ride height of the car determines where the sub-frame needs to go vertically in the car. Picture the sub-frame sitting on the floor on stands at the correct height and you raising or lowering the car onto it to the desired ride height. Hopefully the front crossmember falls in a location where it can be tied into floors, rockers, and subframes without too much drama.

That takes care of height. Then you have determine where the wheel/axle centerline will fall in the wheel opening. Assemble the housing and bars to the sub-frame and get an axle centerline to front of crossmember measurement. Some builders will give you a measurement it was designed to have. Keep in mind, the whole assembly can be moved forward a bit to shorten the wheelbase some, improve weight distribution and allow for the only the front edge of the wheel openings to only have to be moved for big tires. That determines where the crossmember will fall horizontally between the rockers.