Maneuvering speed and rough air speed are different
animals. For the curious, refer to JAR-22.341, which
begins with 'in the absence of a more rational analysis..'
and continues with an equation. The equation involves
a number of parameters, including the slope of the
wing lift curve and the wing chord - so it will vary
with sailplane model. There is no obvious relationship
between the rough air and maneuvering speeds, but JAR-22
also requires that the rough air speed must not be
less than the maneuvering speed. Of course, not all
sailplanes are subject to JAR-22.

http://www.tux.org/~milgram/temp/jar22.pdf

Ian