I'm used to relieving a flathead cylinder (different purpose, obviously), but we normally stopped the relief depth at 1/16" minimum above the highest point of top ring travel, to prevent the ring from getting too hot. They don't appear to suffer any damage from passing right into the relief, provided it's not too wide. If you have to do this, be sure to make a small radius at the edge to help the ring jump in and out (like a 2-stroke exhaust port).
A common recco for valve relief used to be a minimum of .100" larger radius than the valve head.
Depth to whatever is indicated (1/32" is pointless - but it makes you feel better! some engines will take 3/16" deep.
Shape: by moving the 2 cardboard circles (bore, and valve @ OD + .200") away from each other until the crescent of overlap is the relief depth; this determines how far along the wall the cut extends. It's going to look like yours, but easier to duplicate in all cylinders.
