I use qwerty42's Excel utility to give me the coordinates for FSET.
I ported that Excel file's formulae to C# so I'm using the same coordinates. To save anyone else time looking at this...
I took my random Level 13 size square that was producing an extra "band" with masks at the bottom.
AeroScenery generated the following coords in the TMC file for that square
<[vector2_float64] [lonlat_min] [2.15 46.77]>
<[vector2_float64] [lonlat_max] [2.20 46.74]>
When I hand tweaked the coords to this
<[vector2_float64] [lonlat_min] [2.15 46.77]>
<[vector2_float64] [lonlat_max] [2.20 46.*75*]>
It worked as expected and just output one ttc file with no masks.
So, it's a rounding error.
Question is, if I fix that does it break it for other scenarios? (Pessimistically I'm guessing yes). It'll need some testing.
The ideal here would just be to be able to lookup "map_13_8188_a608" from a predefined list and get the coords GeoConvert expects.