The sLeftMargin box contains a value which specifies the width of the unprintable region on the left edge of the page. Typically, this value is only set for dot-matrix printers; it is specified in master units.
For the Diconix 150 Plus printer, the maximum printable page width is 6.9 inches. Subtracting this value (6.9 inches) from the width of a page of paper (8.5 inches) leaves 1.6 inches for the the borders of the page. Dividing 1.6 inches by 2 leaves .8 inches. This is the width of the left and right borders. Converting this value to horizontal master units (using the Xdpi value from the master-units dialog box), we obtain the value 768. This is the value that appears in the sLeftMargin box.
For the HP LaserJet IIP printer, the unprintable left-margin measures .166 inches. Converting to master units (300 * .166 = 50), this value is entered in the sLeftMargin box.