The problem is getting hold of a instruction book for the Bronica ETR E bellows and getting the right bellows factor for a particular lens ie: a 135mm lens or a 50mm lens.
Zero bellowf factor is when the lens is focus at infinity. Focus each lens at infinity then compare that distance with the amont of bellows you've used.
I might have the instruction booklet of the bellows. If I do I'll email it to you. A lot of my photo gear is in boxes as I am refurshing a room, so won't able to tell you straight away.