How to put function value at a particular location, in the calculator?

Hi, my original problem involves finding the thickness of fluid jet. I need to find the location of points where the fluid velocity is half the velocity at the axis(y=0). My idea was to perform something like

U = u(xcordn,ycordn) / u(xcordn,0)

, so that the velocity in whole domain is normalised with the velocity at axis, and then later draw a contour line for value 0.5. But I guess we cant give coordinates this way. Is there any way to do this in calculator, or any other idea how to approach this problem?