Vertical tangent curve on surfaces — how?

Maybe I’m overlooking something really simple, but can anyone advise how can I create the red dashed curve shown in the pic — a vertical tangent curve on the surfaces (not isocurves) — so that I can then make the vertical yellow surface?


Hello - Silhouette from Top, (if I am reading the image right) or DraftAngleAnalysis ought to help.



Silhouette worked perfectly, many thanks!!