Hi Sebastian, and yes like Burr mentions your best bet for a base surface is to make a simple plane using Draw solid > Plane - that will make a simple untrimmed plane surface which will be a more predictable base surface.

It looks like in your original file you made the base surface using something like Construct > Planar (or possibly by trimming some other larger surface?), which does build a planar surface but it will be a big plane that uses the curves you picked as trim curves on the interior of it - it's best for Flow to directly make a simple plane instead of that so you have the underlying surface at a known orientation.

In order to see the underlying surface on your original plane, select it and then use Edit > Show pts like Burr mentioned, and also note that you will need to zoom out a ways to see the surface control points because they're a ways larger than the visible section of it which contains the trim curves.

Then the other thing to be aware of is that the places that you pick on both the base surface and the target surface are significant and will control how the pattern is rotated when going from one surface to the other, one of the links I posted above has some more description of that.

- Michael