Just wondering if I need to adjust an exposure pre-dawn or post-dusk.

I'll give a bit of a work flow example to help explain my question.
  • I use Aperture prior with a high ISO and maybe large aperture to get an initial reading, then using a smart photo app, adjust the calculation for ISO 100 and change the f-stop
  • Change to Bulb mode, set up my intervalometer and key in the shutter exposure time and set off the shutter, adjust f-stop down.
    However, if for example it is dawn, and the exposure is going to be for say 2-3min duration, during this time, the ambient light shall be increasing as the sun gets closer to the appearing over the horizon and hence the image shall be overexposed. The initial exposure reading is really a point in-time, but the actual exposure is over-time, possible 2-3min even longer.
  • Is there a way to calculate what time to decrease to exposure by, or is it a guess, best hunch etc. (This exposure time would have to increased during sunset as the light is decreasing)


Thanks

Wilko