This is true, most of what I do is just math. That’s especially true for the rear wheels.
The front is more complicated obviously, and that’s where actually having mounted a bunch of different size wheels (and the biggest ones that will fit) comes in handy. Because the diameter and backspace is not just set by the tire fitting between the body and frame, but also the tie rod end clearance, ball joint clearance, body clearance when the wheels are turned, etc.
