Canon EOS-1D X Mark III Advanced User's Manual page 838

FE Microadjustment
Caution
Normally, this adjustment is not required. Perform this adjustment only if
necessary. Note that performing this adjustment may prevent you from
obtaining suitable flash exposure.
You can fine-tune the standard flash exposure level. Effective if main subjects in autoflash
shooting without flash exposure compensation usually seem underexposed or overexposed.
OFF: Disable
ON: Enable
To access the adjustment screen, select [Enable], then press the <
Standard flash exposure can be adjusted in a range of ±1 stop, in 1/8-stop increments.
Set on the positive side if main subjects tend to be underexposed and on the negative
side if they tend to be overexposed.
838
> button.