Again, any formula you use is going to depend entirely on the optics you use. I repeated my build for my frag tank which is a 33 long (same footprint as a 55) and I only used 24 diodes in 2 pods of 12. The huge difference between the 2 is the optics. On the 210, I used 60 degree and on the frag tank I used no optics. No optics = ~120 degrees. I get great coverage with both. I had to go with 60's on the 210 to get the penetration I needed for a 30" deep tank. If I had gone with anything tighter, like 45's then I would have needed more diodes to get sufficient coverage of the surface. Had I done that, I would have been dialing them back even more. Personally, I would say that a good number of diodes for you to look at would be right around 84-96 for your tank, with 60-80 degree optics. For color mix, you really might want to consider more RB and blue in that mix. You would be surprised how white your tank will look with that mix. As I mentioned, our of 72 diodes, I have 36 RB and 12 Blue and I still personally feel what it is not quite blue enough. If I had to do it over again, I'd probably throw another 4-8 Blue's on the tank.