To make the finetuning easier other light sources were switched off. So I could see the result of this particular light only.
It turned out that proper placement of the softbox relative to a model was the most challenging task. First attempt was to just put a light source right to the model (picture left):
The effect is not fully as I wanted. Obviously, the right part of the model's body is lit, but the contour line along her body is actually not visible. The next attempt was to move the light source a little behind the model, projecting the light slightly from her back:
This is much more satisfactory. The line along model's right arm and head is more visible. Sure I lost some light on her face, but it would be compensated by the other light sources.
The final position of the rim light source became eventually like that: