Do you have a little more information about how this formula was created? It might work out in some cases, but it needs to factor in a LOT of variables in order to be accurate and a simple equation like that doesn't really factor in the complexities of various tanks.
To get a true estimate of how many you need, you have to figure out how deep the tank is and what kind of optics you're going to be using. The narrower your optics, the closer the spacing needed to maintain coverage and color blending. For 12" or shallower tanks, no optics are needed. Up to 20" you should be okay with 80* optics, and deeper you'll want 60* optics or narrower. For no optics, you can space your LEDs as wide as 2.5 inches, so long as you get decent coverage. With 80* optics, no more than 2 inches is recommended and for 60* optics or narrower, you don't want to be a whole lot further apart than 1.5".
So from here, Do the following:
(Tank length / spacing of LEDs) x (Tank width / spacing of LEDs).
If you have a 30"x12" tank and you're using 80* optics:
(30 / 2) * (12/2)
15 rows lengthwise x 6 rows width wise.
This will give you high coverage on the edges and in the corners, so you may be able to leave off 2 or more rows lengthwise and a row or 2 width wise. With our example, that would end up being around 13 lengthwise x 4 width wise, or a total of 52 LEDs.
This does bring up an interesting point though, in the old days of DIY LEDs, the standard was create an evenly spaced array that covers the entire tank. Now, people are getting into the habit of using pendant style lights and having a little darker corners in exchange for cheaper implementation costs. So blanket statements, even based on optics, are kind of going out as far as LEDs go. You can still use them, but it's not always the most efficient.
