your vertical coordinate is Z, not Y... so change the those conections in the definition.
Take your surface and use a divide srf component with 2 division in one direction to have the points that you need... use cull pattern or something like that to separate the list in three lists and use the parabola definition.
For the range, you can use a line between the two extreme points and evaluate the mid point. Make two more lines from the mid point to the extreme points and use the divide curve component in the two lines.. use the t values from the divide component, one will be the positive values and the other the negative values (multiplicate with minus 1). Replace the range component with these values).