If you use a 4" sidewheel and you have increments of 6mm what would the gap be on a 5" wheel if you get my drift?

The curcmference of a 4" wheel in MM is 319.2, diveded by 6mm gives 53.2 equal divisions, on a 5" wheel dived into the same number divisions they would be 7.5mm wide. Simples! Which also confirms that dividing by 4 and multiplying by 5 to arive at the same answer is another method of calculating it. Simon.