Hello,
I agree that EI is much simpler than people give it credit for, however, I'm not really sure where we're going with this. The starter pack provided to you by our kind sponsors, are designed within the context such that product weights are based on the per unit cost of the various salts. It may be that KNO3 and MgSO4 are twice as cheap as KH2PO4, so the supplier has to package the powders that makes economic sense as well as to satisfying the performance requirements.
That is also why it is called the "Starter Pack". It gets you started. After that, you will probably have figured out how much of each powder you tank needs or how much you can get away with. This means you will figure out what to buy, and when to replace the stock.
Dosing is a very personal thing. More dosing produces more growth and more organic waste. This then requires more maintenance. Depending on flow/distribution, lighting and injection rate, a tank may require more of this or it might be able to do just fine with less of that. If your tank suffered chronic BGA, for example, then you would have to consider dosing more KNO3 for that tank at that particular time in it's development. If that tank was prone to GSA, then more PO4/CO2 is called for.
EI is an interactive procedure, so that the numbers given with the pack are good for "Starters". It never even occurs to me that any powder should run out at the same time as others. I can always see when I'm running out of something. If I need a powder, I get it. If I want to be efficient, then the lowest supply of powder is a proxy vote for ordering more of everything. It's very easy, and so really, it's not something we should ever really worry about.There are loads of items to be concerned about when attempting to optimize your tank's performance, believe me, but this is not one of those items.
Cheers,