I've never done a Nikkormat, but the F's FTn meter is not hard to adjust.There are two potentiometers within, one for the meter and one for the battery indicator, and they're slightly interdependent. There may be some argument about how perfect a mere adjustment is at all stops and speeds, but since the F's usual lenses and shutter speeds operate in full stops, and the center weighted meter is not super-precise anyway, I found it was easy to get well within range for film.
For calibration I used a fairly new Minolta X-370 which always delivered extremely good exposures, and since I could set up both the Minolta and a Nikon on a tripod with comparable 50 mm. lenses, aimed at an evenly lit beige wall, I just duplicated the Minolta's readings. To account for any non-linearity at extremes, I set the exposure at a low ISO (probably 100), and a shutter speed and aperture more or less in the middle range. There are likely fancier and more scientific ways to do this, but I got my three FTN's to read repeatably within 1/3 stop of each other and the Minolta, and they delivered good slide exposures for years.
You have to take the leather skin off the finder to get the top cover off. My favorite glue for putting it back is a craft glue similar to white glue sticky when wet, and more flexible when dry. In the US, the stuff I like is "Aleene's Tacky Glue." It holds up well, but is easy to remove without tearing or residue if need be. I originally recalibrated all my FTn's for silver oxides, back when you could still get the large ones.
What I don't know is how the battery box of a Nikkormat compares. The F used two batteries, and the positive contact is not at the end but the side. Because of this, you cannot adapt smaller batteries with an o-ring or plastic sleeve (as you can on, for example, a Pentax Spotmatic), but must use a conductive ring. You can fudge it with aluminum foil too, but for a more secure connection, a fitted metal ring works best. I have seen somewhere the suggestion of taking the shell of a larger cell (obviously not a mercury one) and fitting the smaller battery inside that.
You can also use alkaline cells if need be, but as everyone knows, they don't retain their voltage after a certain point, but if you calibrate for 1.5 volts, and the cells are fresh, they'll last a while, and the battery indicator should tell you when they've dropped.
Although it's pretty simple once you look inside, I used to have a link to a web page with instructions for calibrating the FTn, but I don't think it survived a recent computer crash that obliterated my hard drive. If need be, though, I might be able to find a copy somewhere, as I did back some stuff up.
