Ok, not to be contrary, but I want to do this completely differently.....
For me, since my mono-rail doesn't have a built-in ruler, I would find it much easier to just have a table for *subject distance*. I think I could accurately guesstimate subject distance better than bellows draw (image distance). Why does no one do this? Why is the sky blue?
Because at those distances you can guesstimate, you don't need compensation.
At those where you do, guesstimate errors are big.