Point on line closest to two points outside that line

In partial answer to my own question here is a very informative posts by David (see link below).

Most to the point:
“When Rhino makes curves it typically picks a domain from zero to the length of the curve. There is no stringent need for such a choice, except that many algorithms ‘like’ it when the physical size of a curve is roughly equal to its parametric size.”

and also:
“Certain special types of curves have an intrinsic domain. Lines are always (0,1), circles are always (0,π), arcs are always (α0,α1), where the α’s represent the start and end angles of the arc.”

Thanks again everyone for your help!