Here's an excellent thread from across the street on this very subject:
The easiest way to get flex track t follow a particular radius is cut a piece of 1x1 (or sacrifice a yard stick) and cut it 30" long. Drill holes one inch in from each end, with one hole big enough for a pencil or felt tip marker to stick through and the other small enough that it can be held in place by a brad or push pin. Pin down the measuring stick in the middle of your layout surface so it won't move but can still swing. Put the pencil or magic marker in the the other end and swing the measuring stick in a complete half circle around the table. You'll then have a perfect centerline to lay a 28" radius curve with flex track.
HO Scale Modeler