Measuring diagonally from front to back will get you close. Front left to right rear, and front right to left rear.
Not sure if it is the "correct" way but, I dropped plum bob lines from different spots on the car. Put some tape on the floor and measured the distances between the marks. My problem was that my sub-frame connectors covered up several of the measuring points under my mustang. You can use places like the ball joint in the front, and axle flange in the rear. closer you can get it at home the cheaper the trip to the alignment shop will be.
'90 mustang 545, Weiand tunnel ram, Predator carbs, c-4 with brake, 4800 stall UCC, 9" 4.10 gears, 390/45r 15 drag radials, ladder bars.