The article about portrait photography at night is unclear.

No amount of trouble is accurate. The standard method is bracket exposure or correction after shooting.

Reference exposure value: F4 (1/2s or12s or 2s) is suitable for urban night scenes.

The distance estimation of flash index GN can be used to estimate the distance between the built-in lamp and people of 2-3 meters, without calculation and estimation, or TTL can be used, but the shutter speed is relatively long.

In addition: in fact, there are few calculations on the location shooting site, which is so slow. Even if you have reference data, you should look ahead and master it skillfully in field operations.

Supplementary content for you: you can't use flash, you should shoot with high sensitivity and large aperture, and the shutter speed can only be determined by experience, because metering is not allowed at this time.