I'll make a simple example to help show what you could encounter when checking the deck on your engine. Let's say you are checking the spark plug side of your piston on the #1 cylinder, you first need to establish it is at TDC, and then push or rock the piston toward the intake/exhaust side of the block. Lets say this gives you a reading of -.020(below the deck), and then rocking it in the other direction gives you a reading of -.005(below the deck). Since the #'s are both negative and represent a value that is below the deck surface, you add them together and divide by 2(-.02 + -.005 = -.025, -.025/2 = -.0125 deck on the spark plug side). Now check the other side of the piston, the quench side using the same method as above, you might find that that side is -.015 in the hole. It gets tricky when you rock it one direction and get a positive #(above the deck) and the other number is negative(below the deck), but its not uncommon. Once you check both sides of the pistons on all 6 of them, you not only see a variation from front to back, but also from side to side. It would be the determination of the machinist at that time how much to cut the block, and whether any pistons needed to be cut to equalize the deck to zero or -.005, or whatever is determined to be the target number for your deck height.
