Why use width dimension to calculate height? Because tire manufacturers specify height in terms of percentage of width.
So, you want to drop the nose and raise the rear for "The Look?" Good practice if you know what you're doing, but a bad reason for doing it. Getting the nose down a little (12 mm. works for me) by raising the forks in the clamps and using taller shocks (I recommend 340 mm. for Standard models with 18" rear) will make steering faster and more linear--with proper tires!
If you use weird tire sizes, however, the result will not be good. First off using a 100/70/19 in front (assuming anyone makes that size; I've never seen it) with a 120/100/18 in back would not result in a small change. You'd run out of tread in front in a hurry, and the fat rear would slow your handling.
With shock height of 340 mm. and the clamps dropped on the tubes (experiment carefully with this, making changes in small increments) you'll get good results with 100/90/19 or 100/80/19 front and 110/90/18 rear. If you're going to go to that trouble, don't mess with low performance tires like the Roadrider. The Pirelli Sport Demon and Bridgestone BT45 are both made in sizes you can use; I like the Pirellis because they're lighter and do just as well in the corners. Guys like to argue the point, but it's bad practice to mix-and-match front and rear tires. If you stick to the same maker and model front and rear you'll know that the tires will work together, and that you won't have a difference in carcass flex that can put you on your head.
If you raise the rear and drop the nose, be aware that you'll need to address other things as well; better fork springs, tapered steering head bearings, and bronze swing arm bushings at the very least. Also be aware that any problem in the rest of the suspension (wheel out of true, sloppy wheel bearing, etc.) will become more acute.
Pay attention to what TwoMany wrote re. manufacturers' height and width specs--they're nominal, and the only spec you can fully trust is the wheel diameter.
