TurboPrint & PrintFab

If I scale the aspect ratio of an image in Gimp so that it matches A4 paper, then go to print the image with Gimp-Turboprint, I would think that scaling the image to something like 97% or 95% for example would leave all margins the same size. This never seems to be the case and always one margin will be much larger than the others. Why is this happening?

Is there anyway to fix the margins or limit the area that gets printed? IE. set the actual size of that blue rectangle to one I want?

Thanks,

Cheers,
René

Never mind, figured it out. Simply created a new paper size and set the margins there. Then in Gimp-Turboprint the margins show up and I can scale the image to both crop to the right dimensions along with the margin sizes I've set by selecting this new paper size.

Cheers,
René