>>126857414
a good rule of thumb is half a gauge per semitone dropped

9 -> E standard
9.5 -> D#
10 -> D
10.5 -> C#

If you have not checked out string source strings, do yourself a favor and order some.

If I had to have just one 7 string that I'll retune i'd go with picrel, knowing A# std will be a bit too tight and G# a bit to loose