I think if you use 3/4 stock you get the 1/2 inch part rounded properly and
a flat on either side of the curve. Someone made a bullnose cutter that had
two radius cutters on an arbor with a ball bearing between them so you could
adjust the blades to round over both the top and bottom edge at the same
time.
max

I'm hoping you've done the math and know with a 1/4" radius
top and bottom you're limited to a 1/2" bullnose bit and
your stock is 1/2" and not 3/4"?

If your stock is 3/4" I'm thinking there might be a bit all
ready to go from someone like CMT (at least I see it in my
head).

UA100