The depth range for hobbyist tanks is pretty narrow, say 12" deep at the low end to 30" deep at the high end. By my formula, the 12"-deep tank could get away with 2.5% less light intensity than the 20"-deep reference tank, and the 30"-deep tank would require about 3.3% more light intensity than the reference tank. So worrying about tank depth seems to be a waste of time for most hobbyists, which is really why the old rule of thumb of 2-3 WPG worked. Dimensionally it was incorrect to use a volume measure in the old rule of thumb, but since the depth ranges for most tanks is so narrow, it didn't matter. You could use a dimensionally correct areal rule of thumb like I have developed and either include or ignore the depth correction, or you could include the depth in the tank volume instead. All three methods would give the typical hobbyist very similar answers.
But what about some of the Amano tanks, pictures of which have been posted in this forum? One tank I saw was probably 4-5 feet deep. A 5-ft-deep tank would require about a 14% increase in bulb wattage to give the same light intensity at the bottom of the tank as that in the reference tank.