I've done my T8s two ways. 1) 11" strip down the middle of the divide like you're describing; and 2) 11" strip lengthwise along the bottom/back. After I changed from down the middle, I realized I preferred the 11" strip down the middle over the 11" strip across the back. However, I believe I would have had better results had I gone with a smaller strip across the bottom. In order of my experience/preference:
  1. 6" strip (or smaller - 3 or 4") along the bottom towards the back (4 feet worth)
  2. 11" strip down the middle of the divider (2 feet worth)


As far as how long, I always paid attention to my Herpstat. Once it stopped fluctuating and settled into place, I started using my temp gun to get a baseline hot spot reading and adjusting from there. Typically this didn't take any longer than an hour. I'd make my adjustment and then wait another hour then do readings again with my gun. As long as I was smart about my first adjustment, that's about all it would take to dial it in. The Herpstats are the thermostat of choice for a reason - they're solid as a rock for keeping accurate temperatures.