So today I returned to this tool. And the above talk about the calculations being correct are bogus.
Luckily, I figured out how to calculate the proper pos_x and pos_y this evening. And I'm making it a little more convenient to export the overview for CSGO (which uses DDS files rather than compiled VTF files). Unfortunately, MAXScript has not interface to create DDS files directly, but I'm making it so you can save the DDS from the Render Frame Window.
I'll probably release this in an update this weekend.