Exact formulae are impossible because there are so many factors in the equation. As has been mentioned, you seem to have a balance now. But if it were me, I would want a lower balance, because I am a firm believer in as little light as necessary, and light should always be the limiting factor.
My concern over light is mainly for the fish. Forest fish, which is what most of us maintain in planted aquaria, all (with very few exceptions) occur in rather dimly-lit waters. I posted a video earlier today showing angelfish and
Cardinal Tetra in water that was so dark you couldn't see the fish without a lamp, and it was high noon as the above-water portions clearly showed. Bright light does stress such fish. Baensch refers to
Cardinal Tetra as having a light phobia, and they will not spawn except in near darkness. I expect you get the point.
Like Tom Barr said in a discussion with me about light and algae, one should always start with the lowest light essential for the plants one wants to grow in the tank, and then raise nutrient fertilization--which is not only what we add but what is already there--to balance that light. And back to the formula question, there is none; it is a question of everything balancing, and this depends upon what's in the tap water, the size, number and type of fish, the food fed to the fish, the amount of CO2 from the fish load and the bacteria (more comes from the bacteria in the substrate than the fish), and then the fertilizer added to supply what is missing from the foregoing. Even in my tanks using only Flourish Comprehensive, i have to work out whether once or twice weekly is needed, and i notice it does vary from tank to tank as a result of the fish load, plant species and number, and light.
You mention two T5 HO tubes over a 55g. If these are 48-inch tubes, in my view you have double the light intensity you need. That is very bright light on the fish. I had two T5 HO 6700K tubes over my 115g 5-foot tank for a week last year, and took the fixture back in exchange for a twin tube T8 fixture. The light was way too bright. I could hardly bear to look into the tank, I can't imagine what the fish felt. A good carpet of floating plants helps, but if the light is more than what you need for the plants, light is being wasted which means energy to run the light is wasted, and down the line Excel is being wasted and more nutrient fertilizers are being wasted. Things can be balanced at any level from minimal to very high; which level one chooses will depend upon what one expects or wants from the plants.
Byron.
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