You lay your "eyeloop" or millimeter scale over the glossy strip and measure edge to edge. As you have already noticed there isn't really a nice crisp edge to measure from, so don't try to make it rocket science. 4mm +/- 1/2mm is an adequate standard that most anyone can measure on a standard millimeter scale.

This wouldn't be the first time I've found a procedure in the service manual that was incomprehensible. Any Sharp guys out there? How about the registration adjustment on almost any Sharp? After several attempts at following that procedure I came to the conclusion that it just didn't translate well from Japanese. =^..^=