x2
depends on what rails you use as well as if you make any adjustments to the front springs or other stock geometry ...
For mine I blocked up the sled with skid flat, bolted on a flat plate on the drop bracket that extended to the bottom of the rail, pulled the skid back till track was close to tight with rear axel loose as it could go to allow for adjustment, marked where the plate "fit" clamped it in place and then tightened the track to verify setup. Once everything was good, I trimmed the plate to suit and bolted cross axel mount in place through rails and wielded the drop bracket in place.
No formula as everyone seems to be using whatever is laying about and getting it together by seat of the pants / eyeballing ... if it looks right then in most cases it is!