A 12" dial caliper would get you close but not precise due to the curve on the main bearing housing. But it should be within 0.005 of the actual number if you're careful. The number you get will be bigger than the real number.
Like Andy said. You want to use the outer edge of the caliper that is narrowed down and then find the minimum reading so you know your on the bearing bore part that is closest to the cylinder / square to the deck. That is the hardest part. I have a micrometer and dial bore gauge that I can use to measure the bearing bore, but usually I just look up the bearing outside diameter spec.
I just use the measurement to check if the block has been cut and of the four corners are the same height.
If I want the block decked for a specific piston to deck clearance, I just tell the machine shop and let then set it up.
