LGriffin,
You are using it correctly, it takes experience to get a good reading. A steady hand and a good eye are vital. Take a bunch of measurements in the same place and get the feel until you can come up with the same reading several times in a row.
They do make bore gauges with three legs so they will self center and self stabilize. If you know it's a 292 cubic inch, and hasn't been bored, then you will find the bore measurement in a manual or online. A taper gauge would be the proper tool to indicate the health of the bore, but several good readings with your mic can do in a pinch.
A neat trick is to put a piston in upside down and use the skirt to align the inside mic. Good luck.
Stuart
