Yeah that's the same reactor I use on a tank with about half the volume you have. I usually fill one chamber about 3/8 fill and run it every other month.
With that size tank I'd recommend just filling up the GFO chamber half full for the 1st month, stripping the water of all phosphates in a short period of time is not recommended from what I've gathered. As far as not getting a tumble with a full chamber, where is the reactor itself in relation to the pump? I keep mine about level, I've found that the little MJ1200, though reliable, doesn't overcome head pressure well at all.