Hi rado also one other note - the curve to curve backbone version of Flow is probably what you would want for controlling this more accurately since it is based on matching distance traveled along each curve (or matching percentage traveled along the entire length if you use the "Stretch" option).

The surface to surface one doesn't matches instead by UV parameter space of each surface and it can be fairly easy to have UV space that is kind of bunched or stretched in relation to 3D space which would probably create some problems for you. If you do want to use backbone surfaces instead of backbone curves, using the Rebuild command on the curves that you use to construct the bendy surface could help to give it a more regularized UV space.

But just using backbone-curve to backbone-curve mode is probably best for your case.

- Michael