Improved Fish Weight Formula - Figure out weight with length and girth

The generally accepted formula for figuring out a fishes weight based on length and girth is:

Weight = (Length X Girth²)/800

I recently saw data that showed a better formula is:

Weight = (Length X Girth²)/690

So, for the mathematically challenged (like my wife), here is how it works:

Lets say our fish is a 15" long rainbow that is 11" around at the widest point (girth).

Weight = (15 X 11²)/690
Do the girth squared first... 11² is: 11 X 11 = 121 and plug it in
Weight = (15 X 121)/690
Now finish the length times width... 15 X 121 =  1815
One last bit of math: Weight = 1815/690
The final weight (estimate) is: 2.6 pounds 

I have personally only verified this with 3 fish, but it worked much better than the original.