1 cubic ft= 7.48 gal or so.

So take those amounts and add them to water bottles to get them 100% dissolved.

Add 1/2 or 1/4 or 1/8 of the water to get 1/2, 1/4 or 1/8 dilutions.

Your tank is 7.5 gal or so, lets just say 7 gal.
So look at EI at 20 gal tank.

300mls of water + 1/4 teaspoon of KNO3.

20/7 = 2.8 or ~ 3.

So add 100mls of this per dose.

Do the same for PO4, for SeaChem, and traces are already liquid, so 1.5-2 mls etc 3x a week.

You can make a stock solution of 500mls and make it any concentration you desire, the above is just one simple example that just uses the eI method without any calculations other that taking the Volume of dry ferts, adding to water asnd then dividing the amount of water into equal fractions to match the tank size.

So if you can divide, you can do this very simple method.

Regards,
Tom Barr