That was something bandied about in freshwater, and it was about as useful in freshwater as in marine tanks. Let's assume a 55 gallon tank. That would be 55" of fish. A tang can be 6-10". Based on the "rule of thumb", that would be 5-9 tangs in a 55 gallon tank (or fish of that similar size).
That won't work.
My opinion: decide what types of fish you would like to keep and what type of tank (reef/fish only) and ask for stocking advice based on that. There will be a zillion different opinions on what fish to add. To me, that is a very personal choice. Also, each fish you add will usually impact what can be added after it. It is much easier to not put a fish in a tank than to try to catch one in the tank.