PFRAMES Or OFFSET?

Trying to get the perpendicular vector of a point on a curve.
The shape is a star drawn from two circles. One inscribed in the other.
My problem: i draw a curve between the division points of the two circles which are at a different z-value.
When offsetting I don't get the perpendicular point. When using PFRAMES, the angles are each time different on all axis?
Any help ?

Thank you