Lens – Which EXIF ​​tag needs to be set for the 35mm equivalent focal length?

The day you are looking for is called "focal length".

I see the following with 70D, Sigma 24-105 / F4

Focal length: 80.0 mm (35 mm equivalent: 126.1 mm)

Focal plane ???? Tags depend on the digital zoom

P.S. FocalLengthIn35mmFormat is calculated from FocalLength multiplied by "Scale Factor To 35 mm Equivalent".

P.P.S. Here are the formulas for calculating the scaling factor (since this is a calculated value)

FocalPlaneXSize = SensorWidth / (FocalPlaneXResolution / 25.4) (if size is in inch, otherwise use 10)
FocalPlaneYSize = SensorHeight / (FocalPlaneYResolution / 25.4) (if size is in inch, otherwise use 10)
FocalPlaneDiagonal = sqrt(FocalPlaneXSize^2 + FocalPlaneYSize^2)
Crop Factor (ScaleFactorTo35mmEquivalent) = Full Frame diagonal / FocalPlaneDiagonal

(if someone needs it)