Rotate the crank around on each of the 4 corner pistons and use your depth guage to find TDC. Once your there you need to check all 4. I like to check them over the pin, the piston will rock in the bore so if your checking the deck height on the piston at the block outside and inside side you will notice you can rock the piston. If this is your method, make sure you rock it one way and read the guage, then rock the other way and read it there again. Average the reading. Hint, if you do this method, your average will be within 1-2 thousandths of the reading you get directly by reading off of the piston pin aligned portion of the piston. This is quicker and in my opinion its more accurate. Write it all down!