EXFO FTB-7000 OTDR for FTB-200 v2 - Manual (Page 291)

Maintenance
OTDR 277
Verifying Your OTDR
2. Set the distance range to 80 km (singlemode fiber), the pulse width to
the longest value available and the acquisition time to 180 seconds.
The measurement range using the non-reflective event method represents
the amount of attenuation (dB) between the launch level and a 0.5 dB
splice (which can be detected and measured to an accuracy of ± 0.1 dB).
You can measure it by simply making an acquisition on a fiber with a
known attenuation and a known 0.5 dB splice. Attenuation between the
splice and the launch level is added until the analysis can no longer
measure the splice to within ± 0.1 dB.