Gimp: Rounded Corners Radius

I’ve managed to get something close…still not quite right on the corners. Anyone know what a typical radius should be? I’m just shootin’ in the breeze here, I used 15% for this one.

Gimp Steps:

1) Square selection
2) Rounded corners (15% radius)
3) Feather ~ 4px
4) Fill new layer background color
5) Grow selection 1px
6) Fill new layer with foreground color (border color desired)
7) Layer images so border is ontop (requires transparency on border layer).

Produces something like this:

Any suggestions?

updated: Mountaintop Corners

Pretty simplistic way to create rounded corners, but it works for me :-)