Hi,
Long story short, for some reason I don’t understand, when the angle of the helix is a large number, the start angle of the half mill tool that I first revolved is no longer vertical but must be rotated by an empirical value.
The yellow (1) is the one used in my previous post, the green (2) is the one that gives the correct result.
The end part of the mill is unchanged.
As the rotation angle is determined by trial (thanks to parametric version :)) in order to have just one angle to change in the step named “corrected angle”, I add a new revolve step using the end face of the previous body and revolve back with a new angle.
I also use the concave face to do the tool’spath and move the initial shape to the end to finish the tool’s path.
The result is clean and I believe the slot profile is correct, even though I still don’t understand why the corrected angle 
Slot milling V2.shapr (133.9 KB)
Best
PEC