Bill,
The Plastigauge system doesn't care if the crank is ground .010 or .020 or .0999998 under or over or anything. What you are trying to determine with the Plastigauge is 'clearance' and this is what you are verifying. A crank journal would/should have the same clearance parameters whether the thing is factory stock or has been ground twenty under.
What would be a typical clearance for a Stovebolt main bearing, .0015 to .002 or .003...something like that I think. This is how the little string would mash and compare with the gauge on the package.
Stuart
