If your femur gets anywhere near level at the top of the stroke then your cranks are WAY too long.

This is also the fly in the ointment for your original question. While the knee bend at the bottom of the stroke can be set similar for different crank lengths the top of the stroke will vary greatly for different crank lengths.

What the right answer will be will also vary for different postures. If your sitting upright then your femur getting closer to horizontal might be OK. If you want to adopt an aerodynamic tuck then you don't want it to get anywhere near level.

Anthony