Go to an auto parts place and buy a cheap set of feeler gauges, if you don't have any yet. Cost me $1.99 and I think they're very useful.
I checked the factory gap on my BC, people say it's 0.039, but on mine it was a bit tighter than that at one end, and way wider than that at the other. The rollers were visibly non-parallel. I don't know if that is how they were supposed to be, or if they didn't carefully set it before shipping it, or what, but I set it to 0.039 at both ends using the feeler gauges. The little markings on each of the knobs don't seem to tell me anything, I have to have them in different positions at each end to have a parallel gap. Again, not sure if this is intentional or if there's something weird about mine.
In its defense, my efficiency was fine (around 70%+) on the one batch I did with the rollers at their non-parallel factory setting.