I suggest a ratio of 1 wattT5 for every 2 watts halide. This assumes the halides are 10,000K, and the T5s are a 50/50 mix of actinic and blue bulbs. You can get away w/o the actinics though, and use only the blue bulbs, as technically 10,000Ks have loads of actinic (its the blue they lack), and so you could use a ratio of only 1 watt T5s to every 3 or 4 watts halide if you only use blue+/blue bulbs.
I prefer T5s for supplimenting because of the blue bulbs (they peak at 450nm rather than 420nm like a purple/actinic bulb). 10,000Ks already have loads of actinic in their spectrum... any more is for your viewing pleasure, not the corals really. 10,000Ks have more actinic than 20,000Ks even... you just dont see it because of all the warmer colors. What 10,000K halides need is a good blue spike to simulate deeper waters... not purple/actinic. As it turns out, the blue bulbs are much brighter than the actinics as well, and so I consider them an integral part of an reef that is considering supplimental lighting, more so than an actinic.
Thats why I like T5s better than VHOs. Watt for Watt, the actinic T5s will outdo a VHO... the 'other' opinion comes from people who stack a 54wattT5 up against a 110w VHO... they might be both 4', but two rows of T5s is a better comparison.
For 4x400s, I would consider 4x54 a bare minimum and only if you were to run all blue+ bulbs... and with halides that were slightly tinted... like ushio 14,000Ks or hamilton 14,000Ks... not 10,000Ks for instance.
Going for the ideal of 2:1 would place you at something like 14 T5 bulbs, for about 756 watts. You could get away with only 8x54, but then blue+ bulbs should be in the majority, if not only T5 bulb used.
It all depends though on a couple things... what halide bulbs are we 'supplimenting' exactly? And what are the Tank's dimensions?
