I clogged the stones a couple times too. Was a big pain cleaning them (if they spin at all, running some minute rice through the mill with it first set fairly coarse, and then again relatively fine can sometimes totally clean it up without having to take it apart).
I don’t know how to describe exactly how tight you can make the stones without clogging. It’s a different degree depending on what grain you are milling and how much moisture is in the grain. But I have been milling very fine for a couple years now without clogging and I believe that the “secret” is just really paying attention to the flow of flour coming out of the spout. It should be coming out slowly but steadily. If it starts to sputter or stall you have to back the stones off a tiny bit. And when you have them pressed together far enough to get really fine flour, the difference between too tight and just right is a pretty small adjustment.
Like Melissa said, I start the mill with the stones just barely not touching each other. Then I pour in the grains and immediately adjust the stones a fair bit tighter than where they would be grinding against each other if the mill were empty. Having grain in between keeps them from grinding against each other and produces the finer flour. After doing this many times, I know just about how much to adjust the stones, but I am still paying close attention during that initial adjustment to the flow of flour coming out of the spout and I fine-tune the pressure so I see what seems to me like the right rate of flow.
Trial and error and experience. Worth it to me to get fine flour in one pass from the mockmill. None of this is specific to spelt.