Is there any particular reason why you need to invert the PCB image? And how are you producing the mask from the file?
If you're using a printer driver, then you need to make sure that this isn't scaling the output (such as having a "fit to page" option selected).
I regularly use the PDF export option, and import it into a program which does PDF reading, annotate it with my name, design name, logo and so forth, and then print it using a normal inkjet printer. But then I spent a lot of time making sure that the scaling wasn't a factor (however, I've never needed to invert the image)
Is it a photographic process (i.e. you need to provide it with a negative)? If so, how are you producing the photographic material? Is it printed out somehow?
(I'm just trying to gauge why you're having problems with scaling, and where those problems can lie).
