I have a condition where I have a 3 point spline where two of the control point handles are horizontal and the last is allowed to float or remain default.  The spline will inflect between the two horizontally constrained control points but I need to locate that inflection point precisely in one direction (longitudinally).  In the attached image I need the inflection point to be 1/4 the length from the end.  It is free to float vertically as needed so I was hoping to constrain it to the construction line.  I realize that changing the length of the handle at the end will move it but not precisely enough (basically by eye).