Well, yeah... because he's dead! But Sheldon left behind a treasure trove of knowledge that he selflessly shared with all of us via the Internet.

OP: I usually eyeball tire clearance. It's best to leave at least a couple millimeters since trapped debris can get jammed between the tire and fork. This is either annoying or dangerous, depending on the nature of the debris.

Remember that tire sizes are not always honest. You might be able to fit one manufacturer's 700x28C but not another's.