Sxrxrnr- the diameter of the tire is not what is needed here - what you need is the "loaded tire radius".
If you measure from the exact center of the axle flange, hub cap, wheel cover - to the ground and do the correct calculations you will end up with the same information without the bricks or moving the vehicle. Either method does not take in to consideration the growth of the tire as speed increases but I doubt that growth is much at speeds less than 70 mph.