I do most of my black first with the radius set around 30, dialing down to 7-10 for eyes or other detail. Then I switch to white and sort of blast the color around. In other words, instead of drawing steady lines, I center the pointer where I want the most white to be, and tap the mouse button. The I move to another spot, tap and keep doing this until I'm happy.
If I get too much white, I switch to black and do the tap, move process again. I may switch back and forth several times, adjusting my radius as I feel it is needed.
To get rid of solid lines around the edges, I set the color to white, move the mouse a little way from the edge and tap. I move the mouse all along the outside tapping whereever I want a less solid line.