Ok so you're not a SMW but you want to be,
I don't know anybody thast calls the sheet "stock" you must be a machinist
OK post #6
centre of vee to back of vee you say hypothetically 50mm just think about it draw it out. You want a 40mm fold, you say 40mm in from stock? I presume you mean 40mm in from edge so thats 40mm inside? assuming the vee is less than 80mm you can fold it (normal around 25mm) Think where does the punch fall? (top tool) in the centre of the vee. So logically the backstop will be set 40mm back
In reality backstops are tuned to give inside leg dims which words fairly well on the thinner stuff as I said if in doubt use a bit of scrap to test
So you're descibing a Zed section, first fold 40mm inside (43mm overall). Height of Zed 86mm overall
Fold 1 BS = 40mm
Fold 2 BS = 83mm retract + pause before return to 40mm
It don't matter which way the fold goes for bend calcs
we are of course talking CNC stuff here.
Maybe they call sheet stock in your neck of the woods, then keep on calling it that, but then you'd be a stock metal worker
Simply, backstop zero = centre of vee