There's really no way to measure the gap between the clutch plate and flywheel when disengaged...
You'll need an access hole in the bellhousing / scattershield and a feeler gauge. I put one at 6 o'clock on the bellhousing. Rotate the motor via remote starter while under it until a good spot is found to get the feeler gauges in. I use a prop-rod between the steering column and the pedal to hold it at the floor. Takes a few adjustments of my rod under the hood, but its a 30 minute ordeal after the 1000 times I've done it now.
Where are you getting .020 air gap from? That sounds awful small. I run .050-.060 with my McLeod diaphram clutch and organic clutch. Any smaller and it acts pissy between gear changes. With that air gap there is around 1/4 between the throwout and pressure plate fingers..