Can you help with focal length/lens on BMMSC?

We have Blackmagic Micro Studio Camera 4K and our friends gave lens + speedboosters for tests:
1. Nikon G to BMPCC Speed Booster 0.58x
2. Nikon DX 18-35 f/1.8
All work well
If we calculate via Metabones's formula:

18*1,75 = 31.5mm is minimal focal lenght for this lens. I'm not sure cuz this formula for FF but lens is crop (DX)

We need smthing 12mm-20mm with <= f/1.8. What can you offer from lenses?

Aaaaaaaahhhh. You're melting my brain.

You're using a speed booster, on a S16 sensor, with an APS-C lens, and you want to talk in FF/135 equivalents.

So when you say you need something 12-20mm is that FF/135, APS-C, S16, with or without speed booster.

Forget trying to compare a S16 camera to a FF135 Camera, two different beasts!

Instead, embrace the S16 format. A normal angle of view (AOV) lens focal length for the Micro Studio is around 18mm, so without the Speed Booster, the Nikon Zoom is going to give you a nice normal to short tele lens AOV.

With the Speed Booster (18 x 0.58) the 18mm side becomes about a 10.5mm lens, while the 35mm is now a 20, short tele. A 29mm lens is the S16 equivalent of a 135mm 85 portrait lens AOV of about 27-degrees. A 50mm on the SB would be a better Head and should shot as the perspective is more natural looking than the slightly wide 35mm.

That said, using APS/C or FF lenses on a SB alters the lens perspective, so if you need a real portrait lens perspective, a MFT Native 42.5 or 45mm lens is better. But the lens choice you make all depends on what you are shooting and the look you want.
Cheers