The AFFTA standard for fly reel feet calls for a 0.350 radius on the bottom. It seems strange that they chose this decimal fraction of an inch; why not 11/32 so the radius can be cut with an 11/16 ball end mill that you can buy? Or perhaps 18 mm, which could also probably buy.
At any rate, the largest end mill shank that I can use on my Sherline mill is 3/8, so I have to rough out the sole with multiple passes of a 3/8 ball end mill, then sand. There is a geometry problem involved in figuring out how deep to make each pass.
I have both a 2 flute and a 4 flute 3/8 ball end mill, and I have just discovered which is more suitable for the hollowing task. Normally I would expect to use a 2 flute end mill on aluminum. But in this case, the four flute works better. There is a tooling hole (0.194 diameter) in the center of the foot, and the end mill has to pass over it. Here is a detail of two soles, both partly sanded.
The 2 flute cut is on the left and the 4 flute cut is on the right. In cross section, the two flute end mill has different area moment of inertia on two axes, whereas the four flute end mill is symmetric. I believe that this causes a different (and larger) vibration in the two flute tool, disfiguring the area around the hole. The damage seems to be too great to sand out. I could put off the hole drilling until after the sole has been hollowed, but this is inconvenient.
To date I have done the sanding by finger power, using a wood dowel (0.68 diameter) to back up the wet/dry sandpaper. To speed the process, I have now made an aluminum dowel that can be spun on my lathe. Here I have sawn a slot to trap the ends of the sandpaper. There is also a shallow flat centered on the slot.