The PT tendon reverse curve over supports is usually a single-curvature parabola that connects the inflection points on each side of the supports, whilst maintaining the same tendon slope at each inflection point.
If adjacent spans are significantly different (with differing inflection points) sometimes a compound parabola is used.
Things to keep in mind: check with the PT supplier/manufacturer on the systems MINIMUM tendon radius/curvature, and adjust your inflection points to maintain such a minimum. The manufacturer will also have a minimum straight-length distance that is applicable behind the anchorages.
Is this an unbonded PT slab tendon system or a grouted multi-strand beam tendon? The latter typically require a bit more 'finessing', especially if it is a bridge structure.
As Ingenuity pointed out, be careful using ACI/PTI logic of a defined tangent point (typically .1L in USA) to define the profile shape. Especially with larger multi-strand tendons and bonded slab flat duct tendons where severe concrete crushing and bursting can result if the curve radius is too tight. And even with unbounded tendons in members with small L/D ratios as often the resulting profiles are not buildable.
There is no single equation to do it properly using defined radius of curvature for the curve. We do have a solution in our software but unfortunately I cannot give that to you as everyone else has paid for it!
