Where theory meets reality on the Falcon heads is a bit of a problem. Theoretically, you should have 3/16" or so from the edge of the valve to the closest part of the chamber wall, at maximum valve excursion (which can depend on the cam in use).
This is impossible with the larger sizes of inlet valves on the 144-250 heads. You need to scribe around the gasket edge and therein lies your compromise. At some points with the big valves, you will be more like 1/8" (maybe closer) from the edge.
Where your work on the chamber edge blends into the roof, should be a large, smooth radius. You'll need to CC after, and mill as required to restore compression.
Regards, Adam.