Really, the best way is to "eyeball" it, as that's what will be the measure once it's on the road. You want to bolt or clamp everything into place (suspension, axle, wheels/tires, body parts) as close to final (projecte/desired) configuration as possible and preload with some weight (sand/concrete bags, scrap metal, whatever) to approximate final loaded condition. If you don't have the final tires, or are unsure of what whould be best, you can make up cardboard cutouts to various diameters to play with. I would strongly recommend you figure a way to roll the vehicle out so you can get a long view of it from several angles to ensure you're happy with the results. There's no real effective way to tape measure a good look, sometimes what works best for the eye isn't "right" on a tape.