I have used this method, with both success and failure.
First of all, I would say that if you REALLY want to start building something with any kind of serious curvature, dump the bands and go to the hose method. Even if you don't own a compressor, it's not that bloody expensive, and you can inflate it with a bike pump!
But if you still persist on the bands, they should be 2" apart, and damn sure place symmetrically on critical areas (like fadeouts!!) and they should be around 1/2 to 3/4" below the edge of your form, but this is CRITICALLY dependant upon the width of the inner tube you are using. Even though you can get a LOT of pressure to lam' something up, you are WAY more prone to micro air bubble under the glass between the bands. If you are doing wood/wood or black glass, it doesn't matter, but for clear glass, trust me, it sucks to do all that work, pull it off, and see 60 bubbles up and down the limbs that are only a few thousandths of an inch in diameter!!
Take Care,
Marc