Another reducer + backfocus calculation thread

[BeardFace (Nial)]

Hey all, backfocus question for ya.

I've got a .67 reducer that I'm putting on an RC12a. Now the CCD47 states that 85 mm is optimal from the camera sensor to the thread of the reducer, so that's easy, but what does that reduce the backfocus from rear cell to camear sensor to? The reason I ask is that I'm using a secondary mirror focuser so I need to get the backfocus spot on.

Is it just .67 of the backfocus as well? So for the RC12a would that reduce its backfocus from 288mm to ~192mm?

Or is there some other calculation I'm missing here?

Thanks in advance!