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, @-10, then specify 1 11/32" inset, or set the margins @ 1 1/2" and let the printer come up with the answer I guess...
Back to my initial answer, the easiest way is to print a stripe and check, just to be sure...
Nice to know...