I just did a compression test myself. It was my first time, so I was glad that my new tester came with instructions
First let the car come up to operating temp. Then disconnect the main spark plug lead (forget the right name for it, but it's the one not on the distributor) and hook it directly to ground. Then, at WOT, try cranking the car until you get at least 4 compression strokes. If you have a helper, have them check the gage. You are supposed to crank until the gage doesn't go any higher. That is your compression number.
The 235 and 240 seem a bit high, btw.