1/2" is too much clearance from the bottom of the oil pan. 1/4" is the most often recommended clearance. See "Small Block Chevy Performance Volume Two: 1982 and Later", page 129 as just one reference.
There are several different ways to get that 1/4" clearance, including what's been posted already, the clay method, ruler, etc. No matter which method you use, it's important to factor in the thickness of the oil pan gasket. Gasket thicknesses DO differ.
I've used the shim (1/4" nut) method many times in the past and when I do, I always have the pan gasket installed, just to eliminate the torqued gasket thickness issue.