Ted,

Does the rest of the curve look like a reasonable match to the polar? If so that is likely good enough. You are not going to use a speed to fly to fly too slow. You will only fly down to best l/d speed anyway.

If you really want to force the lower portion of the curve you need to use more pairs of points and likely will need to go to a third or fourth order polynomial. The problem is most flight software is designed around the second order fit.

Tim