Hi Stardust, the tricky part is like Brian says - just any arbitrary surface that has double curvature in it can't be flattened to a plane without distortion. Only certain special kinds of surfaces called developable surfaces can be accurately flattened to a planar shape that can be bent back into the surface without any stretching or compression in different areas happening. So surface flattening is a much more complex problem than curve flattening.

Probably my main suggestion is to just draw an approximate plane to use as the base surface and then use that as the base surface for the Flow command.

If you're having difficulty with a particular situation it could help a lot if you would post the 3DM model file with your geometry in it.

- Michael