Yeah... I was a bit off.
We have the margins set at -10.
To be exact, just checked, with 500 mm media (20"), it shows 492 mm, which is 8 mm (5/16") less than the media.
So- The Mimaki will print 5/16" less than the full width, with margins at -10.
And, if you want 1 1/2" margins...