Edgar,
Sorry, I'm really not sure what Corel is doing if it's not using the actual coordinates from the DXF file. I'm sure there is some formula that would linearly convert from the 1:1 scale that Global Mapper exports at to whatever Corel is using, probably just a linear scaling factor against your raster scale. So once you figure out the scale for one scale you should be able to calculate the scale number and use it for other files. You probably also have to offset the top left or bottom right to be at (0,0). In general working with mapping data in non-mapping application is not much fun!
Thanks,
Mike
Global Mapper Support
support@globalmapper.com