E.g. in roll_region function:
<pre>
if ((abs(c_x/REGION_TILE_SIZE)+abs(c_y/REGION_TILE_SIZE))<=starting_area_size) then
return false
end
</pre>
This describes a diamond-shaped area though (not a circle or a rectangle), which may seem pretty strange to the player. It's not very noticeable when starting area is quite small, but if you use larger settings, it's quite apparent (for instance, biter nests can appear much closer to your starting position on diagonals).
I suggest changing this (and the similar block in roll_chunk function) to
<pre>
if (((c_x/REGION_TILE_SIZE)^2+(c_y/REGION_TILE_SIZE)^2)<=starting_area_size^2) then
return false
end
</pre>
which will describe a proper circular area.