Pound per gallon rules are stupid & totally meaningless. (sorry Chris)
As an example, a 20gal tall quite obviously doesn't need as much gravel as a 20gal standard. The footprint is much different & the difference only becomes bigger as the volume of the tank gets bigger.
You can only go by the footprint of a tank to work out how much gravel you need & even then it's a bit of guess work as different decor will impact how deep the gravel ends up being in the tank. If you want live plants you need about 3" of substrate material. If you don't want live plants then 1" should be quite enough. Once you add in the 10lb of gravel you have, you should be able to make a pretty accurate estimate of how much, if any more gravel you will need to achieve your desired depth.