#pixel spacing

For given width, height, magn, how can I obtain the #pixel spacing? For a square image it seems 4/#magn/#width, but for other aspect ratios I can't figure it out.

After digging into one of my old formulas, I found this. Check it out and see if it's right:

if (4 * #height < 3 * #width)
    pixeldim = 3/#magn/#height
else
    pixeldim = 4/#magn/#width
endif

Thanks, that works.