Fine line feed problem with dot matrix printers

I'm writing an app to print on a label that's 3/8" x 1000". It's used to
mark electrical terminal strips. Different terminal strips can have
terminals of different heights.

I'm using a "Fine line feed" command. This command can line feed by n/216".
If I send the value of 108 the line feed should be 1/2". The actual distance
is slightly more (1/64" or so). The main printer is an Okidata ML 590 24
pin. I have also used a Panasonic KX-P1124i with the same results. If
checked and rechecked my source code (which is rather small) and everything
seems good.

Any help will be appreciated.