BB1, BB2 designations etc are not official, but were generated by the playerbase long ago. They are not something considered by SDE or part of any official structure. You just kind of learn which is what over time.
All BB6s and CV6s are lvl 120.
All BB5s are lvl 103.
All BB4s are lvl 89.
Other than that, you can possibly consider BBs between 70-80 as BB3, 60-70 as BB2 and 50-60 as BB1, but that isn't going to apply to every BB. CVs also work slightly differently, with some nations not having what is equivalent to a CV1. SS work more differently still, with the SS1s being much lower level than BB1s/CV1s.
You'll figure these classifications out yourself in time. Event ships confuse matters further.