Had a chance to go all " Walker Evans" on my 4 1/2 projection lens. I noticed that the Bocha Swirl ( BS ) had a noticeable useful range. I Used Whatever subject To Film plane ( WTF ) distance as a baseline. When background is less than 2WTF behind subject, all you have is WTF. From 2-5WTF we now have a useful Background to Film plane Distance ratio ( BFD ) Seems a universal that 2-5 WTF's, ends up being a BFD.
I propose the following formula for those that know what bokah is, but don't know how to get it. Expressed as a ratio between camera, subject, and background.