Welcome to the forums...
Elise has the method correct, but you might mean a different 20mm diameter - you might want the edge of the cut to result in a 20mm diameter. Like this:
It depends on what you want...
You might also try a verticle center axis of revolution. This might be more easy for you to deal with. It depends on your geometry and your goal...
Hope this helps.
Thanks for both answers but answer two was what I was after.