the WATTS per Gallon is WHACK
There is no linear way to scale that 'calculation' to fit all sizes/shapes of tanks...
The better way to determine, (empirical evidence), would be to measure the light amount at the depth you plan on placing corals, (the bottom of the tank), and see if you have enough light penetrating the water...
since you dont have a light meter we have to use some 'guesses' and suggestions..
WE can suggest something other then what we think, but when you start having issues and not growing well, you will turn around and say, you were told, or you heard this or that...
t5's are more energy efficient, and dont transmit as much heat to the water as metal halide, but, a good 70w-150w metal halide setup will grow everything you can imagine, and you wont have a question about whether or not this needs more light then the t5NO light can give or whatever...
go ahead and start up your tank, even before your holiday break.. just run water and sand and rock and power head.. and a heater or something to keep if from freezing over night, while you are gone...
those two weeks head start will mean the world when you are cycling and waiting... and looking at an empty tank!!!!