Aside from running grain through trial-and-error method, does anyone know a effective gap for a two-roller mill?
The best way is to use a set of flat feeler gauges. The proper gap for you depends on your malt, your lauter tun, etc.
Make sure that the gap is consistent across the rollers--I've seen mills where the gap is uneven from one end of the rollers to the other end.
There isn't really any magic number you can set your roller gap to. It really depends on your malt grain size distribution and your mash/lauter system. A good set of flat feeler guages will allow you to get a consistant gap across the rollers. Check both ends and turn the rollers a bit to make sure they are true to each other. Mine are slightly bent and have a varying gap as they turn. My current setting is about .033 - .035 inches. I use Crisp 2-row as my base malt.
Hope this helps.Steve
Another factor to consider is the diameter and knurl of your rollers. The larger and smoother your rollers are, the tighter you can generally mill your grain without damaging your hulls too much.
If you're unsure if you current setting is working for you, try a slightly larger and then a slightly smaller mill gap. If you widen you're gap and don't see any loss of extract, widen a bit further, etc. When you've gone too wide you'll see a drop in efficiency.
Conversely, mill a bit tighter and try the grain in your system. When you start having run off problems (or stop seeing improvements in your extract), you're milling too tight.
One important question: do you have an effective way to deal with a stuck run off if it were to occur (like lauter rakes)? If not, definitely err on the course side rather than flirting with disaster.
As others have said, there is no one right setting (especially when different malts are involved).