In the calculator function, you have the functions Radiograph_grid and Radiograph_mesh that give you the integration of a variable along a ray cast. In your case, you want to get the Max. of a variable, not its integrated value. This is possible in EnSight version 10.1.X, not in prior versions. To do this, define the environmental variable ENSIGHT_RADIOGRAPH_OPTION to 2. Then, open EnSight. Now the Radiograph functions will return the max. value instead of the integrated value of the variable (if you set it to 1, they'll return the min. value).
Note that the Radiographic functions are quite computationally slow. You might want to try this first using the Radiographic_grid function with a small number of points to sample, to check if the result is what you'd expect before doing the entire correct calculation. See The Help -> User Manual -> ch. 7.3 for the details on these functions.
Let us know if you have more questions or need more instructions on this.