vlfflq so there is no real simple answer to your question. However we can expain you a few things/
Firs of all, when determining what the maximum widest focal length you can use, you have to determine three things
- what aspect ratio will I master in
- what sensor height do I shoot in
- Do I use a lws rod support without step up or step down ring, and do I have pancake design lenses to maximize focal length width
most people shoot 2.39 to master in. the blackmagic has an aspect ratio of 1.78. multiplied by 1.5X anamorphic squeeze you will get a 2.67 aspoect ratio natively on this camera. so If you wand to finish in 2.39 you need to to crop of the edges.
On a a camera with 24mm height you can go roughly 45mm pancake the widest without vignetting (when mastering in 2.39). The bmpcc 6k is 12.99 sensor height. Thats a crop of 1.85X over the 24mm sensor equivalent. So on a bm 6k you can shoot 45\1.85 as widest lens which is a 24mm. If you can find a short build 24mm, (close to pancake). and you use a rod support, so the distance Aivascope to lens is as small as possible than 24mm is possible, when finishing in 2.39. For 24mm you can try many lenses, and 25 also exist, its a comon focal length, try a simple 24mm nikkor ais. they are nice, and sharp and affordable, and small.
Good luck!