We've always gone by the Gallon method - 2 gallons of GB to 1 Gallon of FT. Also, there are many opinions on a formula for a minimum of x# of gallons per lb of fish as seen here Clik
We always used 1 gallon per pound as a rough guideline - when they are younger, that works fine, but as they grow older and without expanding our growbeds, we might go as much as 4 gallons per pound depending on the amount of ammonia, etc. produced - and, as has been said, the key is filtration/oxygen - If you don't have good enough filtration/oxygen supply, 10 gallons per pound may not be enough

Since I mentioned oxygen, there was some posting about oxygen a couple of years back - I had found an article that showed that water dropping into the FT provides better/more oxygen content than air stones. For that reason, we bought a small fountain pump and created a fountain in the center of a pool as mentioned above - no airstones - and never saw any fish coming to the top of the tank for anything but food - if you see them coming to the top of the tank/pond, they may be starved for oxygen