OK ! Thank you very much this time I think I understood.

So I memorize 2 important informations :
1) to cut with bolean for making closed surface where the cuts are done
2) to be perpendicular with fillet to allow symetrical filleting

Last (but not least be sure ;) do you know a way to positioning a surface exactly perpendicular to the camera (our eye) within the 3D view ?

I know I can take a side view and different tools as construction lines for positioning exactly an object, but it would be so cool to have automatically a surface perpendicularly positionned in front of our eye in 3D than I guess Michael already thought to that, isn't it ?

Edited few minutes later :
in attachment there you will see my issues : the ellipse does not close its bolean cut and the line does not cut very well the base.
Could you tell me where you think I am wrong please ?

Cordially,
Guy