Eric - Another way to get a smooth curve is make a template, out of fairly durable material (heavy cardboardboard, 1/4 masonite, etc) of the inside of the ties. It should cover, oh, maybe 1/4 of a circle, and it's might be helpful if there was a hole where the centerof the cuirle is. Roughly like the diagram. Then you can basically "wrap" the flex around your template, secure it, rotate the template and repeat.
Is there any way you can go a few inches wider at the ends of your layout, to allow the "turnarounds" to be a little larger radius? At any rate, go buy or a borrow a copy of John Armstrong's Track Planning for Realistic Operation. In it he describes a fairly easy way of making transition curves. A transition curve starts out very large (in radius) and gradually tightens to the radius of the desired curve. It will give you a better appearance amd better operating reliability as well, especially if you're going to be using as tight of curves at it appears you are.