Sony FX 6 Quick Start Manual page 109

Cine EI: Slog 3 exposure with the s709 LUT
With the s709 LUT applied, you can judge the exposure by eye in the viewfinder.
For more accurate exposure, follow these instructions from Alister Chapman:
expose a white card at 77 percent or
expose a gray card just below 50 percent.
(Chapman says 41 percent for the gray card but I found this too bright.)
To make these values appear as a line in the waveform monitor change this setting:
Menu/Monitoring/Zebra/Zebra level 1 (see next page).
The above levels are for a proper photographic 90 % white, 18 % gray card.
White paper is brighter than a 90 percent white card so expose white paper at 81 percent.
These numbers only apply to the s709 LUT. Different LUTS require different exposures.