Returns the length of the curve's segment between the curve's start point and the specified point
(vlax-curve-getDistAtPoint curve-obj point)
A real number if successful; otherwise nil.
Set OSNAP to tangent and select the point where the line is tangent to the curve:
(setq selPt (getpoint)) (4.91438 6.04738 0.0)
Determine the distance from the start of the curve to the selected point:
(vlax-curve-getDistAtPoint splineObj selpt) 5.17769