AA informs me that the *centre* of the DTAS detector system is 26.7cm downstream
of the upstream edge of the upstream DTAS NaI(Tl) detectors.

The 6x DSSSDs are currently positioned per the 8x DSSSD setup of June 2018, i.e.
the 6x DSSSDs are in positions 3-8 (as indicated on the AIDA snout). 

Detectors #4 & #5 are centred ~9cm from upstream edge of AIDA snout.

This means that the upstream edge of the AIDA snout should be 26.7-9=17.7cm 
from the upstream edge of the upstream DTAS NaI(Tl) detectors.