I voted partially accurate and this is why.
If your going to light your tank with florecents then yes it will give you a 'target' to aim for. If your going to use MH
then no the rule of thumb is useless since a MH
bulb does not and will not light the entire tank with the same intensity as a long florecent bulb.
As a general rule a MH
bulb will light with a good overall intensity a 2' square area. If your tank is only 2' long or less then 1 bulb and the watt per gal
rule might hold some weight. If your tank is 4' long and you put a MH
bulb over the middle then you will have a very bright middle of the tank (minus the shadow from the center brace) and somewhat shaded ends of the tank. TO evently light a 4' long tank you need two mH
bulbs. Since the bulb on the left does very little for the stuff on the right side of the tank its not proper to use its wattage in the calculation for corals on the right side of the tank.
Here is an example
4' long tank with 4 X 2' long 55W PowerCompact bulbs will evently light and provide 220W of light.
The same tank with 1 175W MH
placed in the center will provide 45W less of light (using just raw wattage ratings. In honesty the MH
bulb is more INTENSE than the PC
's) but since the bulb is in the center you really oly have the light over the middle of the tank.
Take two 175W MH
bulbs and put them 1' in on each end so that they evently light the entire tank. Now you have 350W of light over the tank but if you only power up 1 bulb you will notice that side gets 80% of the light whle the other side is hardly lit up at all. Thus the 350W of light is not evenly distributed over the tank its more like having two zones at 175W.
Now. Im not saying either type is better than the other but of the above examples they both have their useage and place depending on what you want to keep.