How snug should the bra band be?

While I have read the "not restricting your breathing" and "not digging into your skin", I just have this general but not exact idea and a unclear cutoff line for a not too tight bra.  I understand that the band is where the bra provide most of the support so it should not be loose but just how tight is enough?

Would anyone be kind enough to explain in detail?  I do get how tight a sock is to my foot so maybe if someone can explain and relate the bra to the sock?

Oh, and should there be any difference for a sports bra compared to a regular bra?

P.S. getting fitted is currently not an option as I am not out.

Perhaps what I've learned will help a bit --

https://www.susans.org/forums/index.php/topic,60295.msg801057.html#msg801057

When choosing fit, measure chest. Cup size would depend on  how big you have determined how big of a bust you want. 2 to 4 inches for most band sizes.(pre-measure with forms with you wish). Nipples should be very slightly above cup centre(for trans with no breast). Back of band just below the shoulder blades. Fit to centre row of hooks, and be able to fit two fingers between skin and band. Straps snug but not tight. (no matter what you do they will slide any way.) keep in mind that different manufactures and styles will measure different. It will take time but  you will get better at fitting as you shop more