The polar angle of a photon from the Radphi target producing a shower in
the lead-glass detector (LGD) depends upon all three coordinates
![$(x,y,z)$](img3.png)
of the shower centroid in the LGD. In the coordinate system with its
origin in the center of the target and its
![$z$](img1.png)
-axis pointing along the
incident beam direction, only
![$x$](img4.png)
and
![$y$](img5.png)
are directly determined from
the data while
![$z$](img1.png)
has to be inferred from the shower energy. In this
note a combination of shower simulations and phenomenology is used to
motivate a recommended way of calculating the
![$z$](img1.png)
of a shower. This
result is compared with the method that is found in the code that has
been in use up to the time this note was written.