Thanks. It seems useful to me too, to look at all of the details. On a fixed mount with full size sensor, the 412 Rule will match limits with what we expect from our Depth of Field sharpness rule (CoC). Which is possibly slight overkill sometimes, but for a DX sensor, that is a 267 rule, which is not much exposure.
What it boils down to is that our widest lens (shortest focal length, 12 to 20 mm) will give the best Milky Way results on a fixed mount.