How to define "Experimantal file slice position" value in the film dosimetry analysis?

Dear all:
I am new to the 3D slicer. The purpose of using 3D slicer is to do the gamma analysis using Film Dosimetry Analysis .
I have created the film calibration curve.
But I am confused in defining “Experimantal file slice position” in the film dosimetry analysis.
Here was my exposure setting:
Total phantom thickness: 10cm, Beam angle: 0, Beam isocenter: depth: 5cm, SAD: 100cm, Field size: 10cm x 10cm, MU: 200.
I placed the RW3 film in the middle of the phantom(depth 5cm) in the same position of the beam isocenter.

So my question is how to define the file slice position? I have tried 173.6 and 19 (the underlined value in the picture). It dosen’t seem to be the correct value.
Please give me a hint, thanks!