Forget all four, and find the highest point of the piston relatively to a straightedge placed on the deck. The average does not matter, the maximum does. The goal is to ensure the piston and the head do not touch during operation, by ensuring a minimum piston-to-head clearance

That being said I found no significant variation across the piston crown in my engine

If you are worried about deck height variation, that is covered. The FSM instructs to check that with a straightedge, and the allowed distortion limit is 0.10 mm