Milling an intake manifold.

You math is correct. If the manifold lines up with no gasket, you need to machine whatever the thickness of the gasket is off the intake.

And I do not start counting the say .060 that you say your gasket is until I take a light cut or two until the surface just cleans up.

Unless you need to worry about port volumes for some class, I never ever machine the head.

Why? Because of tolerance stacking you can never say the next intake will fit either.

Once you machine that head it’s ONLY going to be correct for a block with that deck height and an intake with that exact width.

You can measure 5 brand new manifolds that are exactly the same part number and such and none of them will be the same.

And the difference can be quite large.

A manifold is far cheaper than heads. I always adjust the manifold and not the heads.