CerenkovPMT including refraction index of entrance window, Quantum efficieny and reflection coeff
of photocathode based on NIM A378,1996. 
All other PMT detection efficiency parameters like dependency on polarization, impact angle etc listed in this paper 
are not taken into account.
 
I have used the local coordinate system of the VDC frame for the position dependency

=> One CerenkovPMT hit == One PhotoElectron 

X: along the long  side of the VDC frame [-105, +105] cm
Y: along the short side of the VDc frame [ -25,  +25] cm


Important Parameters:  (total) Tilting Angle: +20.8*deg (orientation: \)
                               Kink    Angle:   7.4*deg 

=> tentative conclusion: 
   I see a exp(-x) position dependency in the number of photoelectrons.
   Neven reports a ~linear position dependency in the TDR ...