@Floob Thanks for stopping by! :)
The overlays I'm trying to convert are OrionsAngel's.
The only config files I have are the mameconfig and the mamelayout (in the zip)
I have generated the cfg just using the values in the mamelayout file, but they are pretty much all configured with the same values (unless my conversion script is wrong, which is possible), so I'm starting to think the values in the mameconfig file are important as well... but I just can't figure out what I should multiply.
For now I'm resigned to measure the values "by hand" in Photoshop (I have done about 20 of 250...) which will take some time but is not very hard.