Pretty good. Seems to give a good approximation.
Taking it one step further, solving the equation by setting both equal for 406 and 451 yields:
Width 406 = Width 451 + 23mm
In other words the diameter of a 406 and a 451 wheel will be equal when the 406 tire is about 23mm wider. So, say, a 406 wheel with 50mm Big Apples will be roughly the same diameter as a 451 wheel with a 37mm tire.
The general equation is, two wheels of differing ERD will have the same diameter when their tire widths are set according to the following formula:
Width Tire ERD X = Width Tire ERD Y + (ERD Y - ERD X) /2))
I found the following formula to be close enough to the roll-out diameter, which is the one that is used for gear-inches (but not necessarily for frame clearance)
Wheel diameter = ERD + 2* tyre width
e.g. 406 + 84 = 490mm
451 + 56 = 507mm
My Moulton front measured=480mm; calculated
406 + 37*2 = 480 spot on
Moulton rear measured 495mm; calculated
1.75"*25.4*2 + 406 = 495mm spot on