Where to capo for Bb Lydian
Put the capo at fret 5 and play Bb Lydian using F Lydian shapes. That is the easiest of the eight positions ranked below, and it is what most players reach for. Against playing Bb Lydian open, it drops the mean difficulty of the key's seven chords from 4.7 to 3.4 and gains 4 ringing open strings.
You want Bb. Capo 5 and play in F.
All eight positions, ranked
Each row transposes Bb Lydian down by that many frets to find the shapes your hand would actually make, then scores the best voicing of all seven chords — fewer fingers, less stretch, closer to the nut, fewer muted strings. Lower is easier. Standard tuning (E2 A2 D3 G3 B3 E4).
What you are playing
The seven chords of Bb Lydian are Bb, C, Dm, Edim, F, Gm, Am — the ones the ranking above is scored against. Their diagrams are on the Bb Lydian key page.