I was going to answer this last night but both
@Photo and
@rdonson are right in that the correct setting is... what is correct at that time, under those light conditions. There are two ways to approach this...
1) Go to a local camera shop or online, and pick up a basic handheld light meter. Used is fine... basic without all the digital readouts is fine. You can pick up something simple like a used Gossen Pilot light meter for under $50.
https://www.amazon.com/Gossen-Pilot-2-Light-Meter/dp/B0000AG93C/
2) As
@rdonson mentioned, become your own light meter and start experimenting. You do not have to fly to do this type of practicing... Simply have the H booted up and the camera connected to the ST-16. When viewing the screen you will see the brightness change as you adjust settings in Manual mode.
However the screen on the ST-16 is dark, so the correct setting is going to be dark as well. A good general rule is when the image on the ST-16 looks "right", reduce the exposure by 2/3 to 1 full f-stop. So, if the image looks good at a shutter speed set at 1/500 second, change the shutter speed to 1/800 to 1/1000 second. Note that the shutter speeds change exposure in 1/3 f-stop increments. ANNOYING SIDE NOTE: When using the CGO3 app and the ActionGrip, exposure control is limited to full f-stop increments.
The more accurate way to do this is to get a photographic gray card set, but this will necessitate taking photos of the card and then removing the microSD card and checking the photos to compare results with with the original target. So you will need a laptop / tablet in the field... trying to check the exposure off of the ST-16 screen, will be no more accurate than guesstimating as described above.
https://www.amazon.com/Anwenk-Balance-Exposure-Photography-Calibration/dp/B01DPV5PUA/
The grey card provides a standardized neutral grey tone that your camera is calibrated to, for exposure and color balance. With our wide angle lenses focused on distance, it will be out of focus when placed in front of the camera to fill the frame. No matter as you are checking exposure, not focus. Take a few frames bracketing shutter speed up or down... compare the tone of the card to the photos. INTERESTING SIDE NOTE: If you are out in the field in an urban environment, exposure readings off a typical red brick wall, will be within 1/6 of an f-stop from a standardized grey card... in terms of exposure, not neutrality of the scene for color balancing.
Also note, that with the rapidly changing light conditions at sunset / dusk, you should be manually bracketing anyway.