Middle appears to be best for mine. I have one up high near 400w Radium looks a little pale, the best color for me is medium-high light roughly 350 to 400 PAR.
For tanks that have 250w MH or equivalent highest point might be ideal. But that being said, a 400w Halide can yield 600+ PAR up high, which I believe to be too much for a valida based on my observations.