two reasons
1) By removing and reinstalling the indicator you may introduce a new accidental error, and
2) your way is waaay more work,lol
Your method may work, but I like the KISS way. My way, the usual way is a one-step deal with no accidental errors introduced. If you are gonna blueprint the deck heights like this, Your way is gonna burn thru a lot of time.
BTW; to prove your sq bar is straight, measure its height on each of it's 4 faces. It should measure the same.
Also, when the piston comes up to the keystock, it has to hit the key with the highest point of the piston, which may not always be right above the wristpin. This is not real important in your case as you have plenty of piston to head clearance in that area. More important for you will be the flat area , the quench pad. Unfortunately, trying to keep that area parallel to the deck surface will be tough as the piston rocking motion will try to mess up your measurements. That is why I use the area above the wristpin, cuz it will exhibit the least change with the rocking of the piston.
