Michael,
It's important to distinguish deck height, which classically is defined as the distance between the point where the piston dome begins and the top of the cylinder, from piston-to-head clearance, which is what you are measuring with the "solder method."
In my case, it's impossible for me to precisely locate the point where the piston skirt ends and the dome begins, due to the continuous curvature from the top ring land to the top of the dome. But by measuring how far the piston sticks up above the top of the cylinder and subtracting the dome height (determined by JE as they made the pistons) this will give me the reference point of where the dome starts, and it's this point that is compared to the top of the cylinder to determine whether deck height is in spec or not.
The point of deck height is to verify that the cylinder height, shims, spigot depth, rod length and rod bushing bore are all correct-- that the piston is not sticking too far out of the hole. It's also a factor in the compression ratio calculation insofar as there's a small cylinder with a diameter equal to the bore, whose height is the deck height, that's a factor in the calculations. Of course the dome protrudes through this in the real world, but ensuring that it's at the right level is the key to ensuring the right combustion chamber volume at TDC.
With the solder method, you're measuring clearance, but there's nowhere to plug the clearance into a repeatable formula to determine volume at TDC. You would have to know the contour of the dome, and the contour of the head, and the space between them at all points, etc. So this is a practical, if somewhat theoretical, calculation to help you figure that out.
I'm probably going to use the solder method also, but to verify that clearance is adequate. . . you never know if the pistons were made after the CNC machine was out to lunch on a Friday. . .
