In reference to FIG. 10, in a manual mode of operation, the user can manipulate, via a pointing device or other means 322, the placement of a marker 330 upon a velocity profile curve 312 in FIG. 8B. The horizontal coordinate of this marker 330' in the velocity profile curve coordinates of FIG. 8C corresponds with a point 330 (Xg, Yg) along the line-of-study 312.