EXFO OTDR 2 - Manual (Page 130)

Analyzing Traces and Events
122 OTDR
Viewing and Modifying Current Measurement Settings
5. If you want to modify the parameters, enter the desired values for the
current trace in the appropriate boxes.
OR
If you want to revert all settings to their default values, tap the Revert to
Factory Settings button.
Note: The modifications you make will only be applied to the selected
wavelength, except for the detection thresholds.
Note: You cannot define a different helix factor for each wavelength. This value
takes into account the difference between the length of the cable and the
length of the fiber inside the cable; it does not vary with wavelengths.

Unless you are absolutely sure of the different parameter values, revert
to default values to avoid fiber setting mismatches.

If you already know the IOR value, you can enter it in the
corresponding box. However, if you prefer to let the application
calculate the IOR value as a function of the known distance between
span start and span end, tap Set IOR by Distance, then enter the
distance value.