Hi Pilou, I'm sorry I'm still not very clear on what you mean.

But you seem to refer to the area where the fillet is going to be placed?

You should not attempt to draw that area directly - instead let the fillet be calculated to fill in that spot so that it can be automatically be created as a smoothly connecting piece.

You can adjust the shaping of the fillet by a couple of things - adjust the radius to be either larger or smaller as desired, and also with the Shape: G2 Blend option in Fillet which then gives you a "Bulge" slider that you can also adjust to tweak the fillet's shaping.

You seem to keep on trying to build a smooth piece without making a fillet, that will be a lot more difficult - the easiest way is to let a fillet be calculated to create a blending area between the 2 surfaces.

If you open up my example model, you will see that there are just 4 curves that define everything, 2 for the top surface and 2 for the side surface.

If you want to adjust the shape of the result, just edit those 4 curves to adjust things as you want.

Maybe if you can explain a bit more about how my example is not right for you it may help me to understand, right now I don't really understand why that does not work for you properly.

- Michael