Dimension precision is set by model not by the individual dimension in SketchUp. You could manually edit the dimension to type 207,5. Normally all dimensions in the drawing should have the same degree of precision to avoid confusion.
Also, if you manually enter text the dimension will not be associative. That is, if you change the edge marked by the dimension the text will not adjust to match. Dimension objects don’t have individual precision settings, so there is no way a Ruby extension could do any different.