I would like to make a sketch plane than is parallel to say the Front plane but tangent to a 3D-curved line.

Note that this is not a curved surface but a 3D curved line.

Basically this positions another plane parallel to the Front plane at the point on the 3D-curved line that is farthest from the Front plane.

It's difficult to follow exactly what you're trying to do from just the description, but I'd suggest editing the sketch and adding a construction line that's tangent to the curved line at it's farthest point, and use that construction line to help define your plane.