Putting aside all intake manifold fitment issues, most people would tell you 0.060" is the max. However, a few years ago we had a member who had milled his open chamber heads far enough that they became closed chamber heads. I think that was 0.080" or 0.090". As I recall he opened up the top side of the ports in his heads and didn't cut the manifold.
R.
I've seen 0.090" too. I assume he enlarged the bolt holes too if he did not mill the intake port side of the head or manifold.
The 0.00123:0.001 ratio is correct.
