If using a cog belt, you need to know the number of cogs per rev and the distance between cogs, try this:
Put the belt around the pulley and put a pencil mark on the belt at the 12 O'Clock position on the pulley, zero the counter and start pulling the belt over the pulley. When the counter gets to 100, hold it still and put another mark on the belt, again at 12 O'Clock. Measure between the marks, close as you can, then divide by 100, that should give you distance per pulse.