Think twice before buying Atlas flex track. It is really difficult to bend. Any curve smaller than O-100 is almost impossible. It always wants to return to straight.
I've been posting on another thread lately about calculation of easement curves. The photo below shows an example of calculated easements that transition smoothly from tangent tracks to a constant radius curve of O-31 sectional track. To avoid problems with the low flexibility of some types of flex track, I suggest using the longest possible easement, which makes it easier to bend, to join with a constant radius curve. This diagram is an example, but the easement length and curvature can be adjusted for any angle and sectional track radius - say O-72. The plot shows centerlines of the easements and sectional track. Meanwhile, back to discussion of track prices...