Using a business card as a gauge gets you close and is inexpensive, but China made run out gauge can be had for $35 and make the whole process simple and very accurate. Markd51 method is a proven winner.
VPI owners.......
If you have the platter that has the three leveling set screws, ( mine is the HW-19 ) here's a free and accurate way to level/true the platter. Cut a business card on a gradual slope. As you slowly glide it under the platter while spinning, you'll hear/feel the contact. It's good to mark the point at which you're adjusting with a tape dot so you'll be adjusting from the same point each time. Sit back and enjoy.