025 to the left and widen the keyway and then step over to the right and do the same to net a. 200 and make the first pass to the depth of the key wayy with multiple (.010 per stroke)strokes and then step over. If you wanted you could make a cutter that was about. I would think that unless your key is to be cut into a bore that is longer than say an inch, flexion should not be an issue.
A cutter might be ground out of the shank end of a HSS single-end end mill.
250 without having to use an undersize cutter and stepping over should be pretty easy. Here is a video (begins with a minute of stills) on showing a method similar to what you have tried.
