I have 3 Mitras on my 72x30x30. Started with 2 hanging parallel, but with all my large SPS colonies that definitely wasn't enough. The three are now are set up perpendicular and they do a great job but still have some shadows from the colonies so I am debating a fourth. I would agree with Steve when he says 3 to 4 depending. If you still plan on stopping by one of these days you'll be able to check mine out.