Usually, most places use H and T. You always count from the center of the mains in the middle of the racquet going outwards. So, start with the middle two mains and that's 1 on each side. When counting out, it's 1, 2, etc. If the racquet has 16 mains, that's 1-8 mains on each side and by skipping 7 & 9 (like on my Wilson), that gives you 8 mains on one side (i.e. 1-6, 8, and 10 grommets used).
Hope this makes sense. As for what the racquet calls for. Go with the manufacturer's specs. If you can't find those, if you have another identical racquet, use that as a guide or look at the old strings before you cut them out and take a pic or note the skips on the mains. Also, looking at how the grommets bend (unless they're new) will also give away the pattern as well.