PROJECTGEOMETRY: When projecting 3D edges onto an inclined face

[indiancad]

I was considering a situation where existing edges from one part of a 3D model need to be projected onto an inclined face of another part. The difficulty is choosing the correct projection direction so that the resulting geometry lands exactly where intended.

With angled faces, projecting along the wrong direction can produce a result that looks reasonable in the current view but does not match the intended geometry.

For those who use PROJECTGEOMETRY, how do you control the projection direction when working with inclined faces? Do you normally project perpendicular to the target face, use a specified direction, or choose another method?