I make the knob slightly oversized (1/2"). The recess is made by file and smothen with sandpaper. I rout the drawer front after fitting. The knob is sanded to fit before gluing and made flush with a small plane.
I make some final sanding and although I really try to sand the dark and light wood separately some micrograin of the dark wood contamin the ash and it looks dirty...
The other photos sent have too much contrast, the one below represent the colour better.