This subject comes up ever so often and needs to be revisited for the newer boaters. It is like everything else, there are many opinions and many different ways to do the same thing. When this happens, I try to keep the system as simple and reliable as possible for me.
I have all of the different mics and measuring tools to do this job, still I use a product called "Plastigage". It can be purchased at a very reasonable price at most automotive parts stores.
It is simply a very precise soft plastic thread that you cut off a short piece, place it on top of the piston between the head and piston. Torque the head down to your specs. Then rotate the engine over one time, remove the head and measure the width of the flattened plastic thread and compare it to the scale printed on the paper that the plastigage came in. This is a very exact measurement that can be used with any piston crown shape whether tapered or dished and any squish shape on the head. This to will help if you are using graduated squish angles. It can also give you the exact squish angle between the two.
The Plastigage comes in different sizes to measure different clearances. These different sizes are color coded as to which thickness range that you want to measure.
Just my way of doing this procedure,