Jackson... kind of lost there with what you stated... my rule of thumb, as mentioned earlier covers spread and intensity. Ex: 220gallon tank @ 5 wattas per gallon = 1,100 Watts. A typical reefer would look into purchasing a 72" fixture. Standard fixtures usually come with for that size tank with 3, 250's MH with supplimental actinics of 96Watt PC's or T5's. (Total = 942Watts) So if you divide the TW(total watts=942) by the TS(tank size) you get 4.28 watts per gallon. Again 3-5watts per gallon rule.
We can even go with smaller tanks and it's pretty much the same thing with the fixture's that the Manufacturer makes. Like the 2 250's with four 65watt Pc's or T5's. There's a rule. That's why these fixtures are made.
As far as intensity which equals lum's, candella, etc etc... the Bulbs are usually 10,000k. (White & Red Spectrum). The supplimental PC's or T5's come in Actinics(Blue Spectrum)to reach further down the tank and simulate the 420- 460nano's rating index.
Spread... well thats more complicated and factors are involed in this equation.
Efficiency..... Hell NOOOOO !! Not MH's !! But excellent growth compared to T5's. Which is faster for growth?? MH's, or T5"s... My opine is MH's.
Heat..... that's a funny one. Why did you have to mention that. Again that was included as part of your opine and mine now. We all know how much MH's can raise tank temps, especially in the summer. But winter time they can prevent your tank temp from dropping during the day during the cold winter months.
Now you have retro's and DIY's so there are equations and other things involved in that aswell.
I agree to an extent that it depends on what type of lighting, aswell as what you plan to keep in your tank that determines what lighting you should get aswell as how you are doing with cash flow.
Like I stated earlier.... my rule of thumb that I follow.
Ask what my tank looks like to thoose who have seen it in person. Thus my rule...
