@fchong
I agree the dimensions are a little tricky to define a circular button. I appears that the radius value is really a diameter value (checking w/ dev team to understand this better). Here are some hints for creating a round button:
0) set the button.height = button.width (i.e., a square)
1) border.radius = button.width – (2 * border.width)
example:
width = 75
height = 75
border.width = 1
border.radius = 73
if border.radius > width – border.width then the low level webkit radius rendering will fail and a square will be rendered.