Unfortunately there’s no real way to objectively reverse engineer the brightness of a light source from a photo without tons more information. In this case I think it’s save to say the light is coming from a window, so your thought to use an HDRI is a good one! To bring it to the right value, add a sun lamp that’s about the brightness you’d like, match the HDRI to that, and then delete the sun lamp. To add a flash, add a small point light just above the camera like you already thought of. You can look up the value of a camera flash in Lumens and use that if you’re using the addon and want to be precise.
Hope that helps! Sounds like you’re already 98% of the way there.
