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You can't really use a per gallon rule when calculating substrate... For example, enough sand for a 20 gallon tall wouldn't be nearly enough for a 20 gallon long.

I use soil substrates to provide co2 and nutrients, but many people have good luck with flourish comprehensive and root tabs without the extra carbon source. In a stocked tank without major surface disturbance, there's a lot more co2 than people expect. As long as you don't pick any "hard" plants (like small foreground plants and red plants) you'll do fine without added co2.

Byron's guides on the planted forum index are very informative and might help you decide.

 

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On the nitrogen comment- plants need all their nutrients available at all times.

I don't know about your balanced systems, but mine (before soil) had all 0's for nitrogenous compounds, and growth was slow.

Most frets we reccomend here (the comprehensive fertilisers) are, for all intents and purposes, trace element ferts. Their Npk is negligable, and they mostly provide nutrients that are lacking- like copper, iron, silica, and boron.