Rule of thumb is:
Find the width you’re comfortable on, either by trial and error or by measuring on the specialized/Selle Italia/Fizik measuring kit.
Then the need for a cutout and the size of the cutout is mostly down to your flexibility, specifically your hip flexors. A lifestyle of doing desk work, watching TV and cycling typically shortens them (because cycling doesn’t stretch them like walking/running/or pretty much any other sport does so actually compounds the problem). Tighter hip flexors pull your pelvis forward (anterior tilt), which pushes your notcher down/back, and puts pressure on it.
Spend all day walking and sleep on your back, probably ok with a flat saddle, sit at a desk, ride a bike and sleep in the fetal position, probably need a cutout of some description.