Keep in mind that not all monitors have the same brightness - even the same model. I’ve had three LG5Ks (as I wanted to check the backlight for evenness) and one of them was noticeably brighter than the other two (the one I kept! 
)
For me I am usually between 50% and 75% or up to 100% for some video sources or if it’s very bright outdoors… but it should depend on the brightness of your room and what’s comfortable for you.
If you’re finding things uncomfortable, or too contrasty, it could be a sign of underlying health issues (or even something like borderline diabetes):