From a technical point, I believe the distance of approximately 200 feet is fine for radar, actually in most situations as the distance decreases, the angle increases, which increases cosine errors which tends to favor the driver (not
As to your concern that your speed was actually recorded in a 40mph, as opposed to the reduced speed zone, that's a tough call.
Did you have a radar detector operating? If so, did it go off while you were in the high speed zone? Typically radar will acquire a speed within ~.3 seconds, I believe.