James, using the EI method doesn't require calculating dosages per Chuck Gadd's calculator or any other. The method is based on estimating the maximum amount of each fertilizer that could be needed for a given size tank, dosing that amount, knowing that even if you are dosing too much you aren't going to do any harm, then doing weekly big water changes to prevent any accumulation of any of the fertilizers. The estimations have already been done, and are in tables ready to use.

I'm not saying this is the only or even the best way to fertilize, but it is an effective way. If you decide to use a method that sets target concentrations of each fertilizer in the water, then a calculator is very useful.

You don't even need a calculator for EI dosing if you premix the fertilizers and dose them as liquids. You just put X doses of the fertilizers in a bottle with X times the amount of liquid you want to dose, shake it up, then dose X amount of that each time. (Chemistry for dummies like me!)