The roll out method is obviously way better than relying on what is printed on the spec sheet. When I did it for the 20" front tires on my recumbent trike the result was 1442. The spec sheet shown above suggests 1580 for a 20" ISO 406 tire. That's nearly 10 percent off my correct input. If you want a more accurate way than the blob of paint on the wheel, tie a narrow piece of cord around the tire and apply some colored liquid such as India ink or food coloring and immediately ride the bike a couple revolutions of the wheel on a cement sidewalk. I generally take the average of three segments and use that as my input. It hasn't changed noticeably as the tire wears since the circumference of the tire (that's what you are measuring) doesn't change enough to make any difference.