Actually its easier than you think. I use a piece of sectional track from Atlas on the radius that you want. Add about a half inch on each side. Draw in black marker your radius onto the board and cut... good news is if you make a mistake you get to cut it again, but nothing else is lost.
I use transitions on my radius so if I have 090 min radius, I start the radius with a piece of 0100 on entry and exit of the radius...