I used to use the SB-21 ringflash with the TTL controller for the F3. I realize that you won't get TTL control with the Nikkormat. There was a version of the controller that used the standard ISO flash shoe also, with TTL control available for the FE, FA, FG, etc.
There are extensive tables that come with the flash, for f/stops to use with the flash (in manual mode) to subject distances, given different film ISO speeds. Also, depending on your magnification, you may have to compensate for light fall off due to the extension of the lens. Some early Macro lenses automatically opened up the aperture as you increased the magnification, in order to compensate. Thus, f/5.6 at infinity is still the equivalent of f/5.6 at 1:2 magnification, instead of f/8. With such a lens, you use the aperture based on dividing the flash guide number by the subject distance.
If your lens lacks the compensating aperture, then be prepared for some rather convoluted math to figure out the exposure. Now you know why people use TTL flash whenever possible for macro use.