Engine milling maths

The flat spot in the chamber next to the spark plug is where they base there mill depth. Correcting if it didnt make the margin was done later or they were scrapped. They knew by each chamber cutting to x amount depth would yield x amount of chamber volume. Rough but quick for production.
I mill the head if possible before the intake, incase I wanna use the intake on something else later...Anyone who says that's wrong will never be machining my stuff. Neither is wrong, one is just limiting the future usage.