SolidKit3D.Math3D.PointOnLinePosition
From Solid Graphics Wiki

The PointOnLinePosition is function inverse to the LinePositionToPoint function. It translates point's coordinates into point's position on a line value.
public static float PointOnLinePosition ( ref Point P0, ref Point P1, ref Point Q );
Parameters
P0  x,y,z coordinates of first point defining the line 
P1  x,y,z coordinates of second point defining the line 
Q  the x,y,z coordinates of a point lying on line defined by points P0 and P1 which coordinates are being translated by the the PointOnLinePosition function to the point's position on line value. 
Return Value
Point's position on the line.
See Also