Lots of questions on even roasting. I personally believe even roasting has a lot to do with what's going on from the tube up, not just the coil down. Of course a perfectly centered heater is important, but I've made this observation before, a coil is not a circle. Just like my avatar, you can see it's not round at all. Yet, it is installed in a perfectly round compartment. Our coil is comprised of 4 parts, wire (leads), mesh (coil itself), butt splice connectors (posts), and disc (mounted to a 7/8 wood disc). This is installed in a 13/16 hole. The mesh is connected to the wire with butt connectors and then soldered. It is then hand rolled into the shape you see below.
If you measure a coil diameter from the outer post it should be about 1 mm larger than the smallest diameter. As you can easily see, a coil is not a true circle, but all of this doesn't really matter. The high volume but relatively low heat generation wafts up in a collective mass, especially as it passes through the fine wire mesh screen that covers the oven. Now the tube and how it's loaded, what it's loaded with, also has an effect on uneven heating. If the contents of the tube is inconsistent (grind, tamp), then uneven roasting may be the result.
I like to keep things consistent, but you surely don't have to. Just load and go, maybe stir once, and you'll get the same end result.
Loading depth is another factor. The further the load is from the heat source, the more the convected air will stabilize before reaching the load. Moisture could possibly affect this, as it easy to see how moisture levels might not be uniform. Of course this is all speculative theory, but after lots of testing and comparing, it is what I've found.
