This may be relevant:
I think I know what’s going on. The issue has to do with the default resolution of MakerCAM not matching the resolution of the file saved from inkscape. Changing the import resolution to 96 in MakerCAM fixed the issue for me.
I’ve created a pull request here to propose updating the file resolution to import into MakerCAM the correct size.
This earlier sled design is actually a little smaller than the new 18 inch sled (I think it’s 400mm?) so don’t be concerned if it comes out a little smaller.
I hope the above paste is a link for you to check out.