Dumb questions about soil and Dr. Earth

1. How do you measure your soil? Like a 5 gallon fabric pot isn't actually 5 gallons. I have some spent soil that I'd like to mix in some Dr.Earth 2-2-2 fertilizer to put my mother's in. I have no idea how much fertilizer to use.

B. How does one go about mixing the fertilizer into spent soil? Does the soil need to be dampened so the fertilizer starts to break down into the soil? How long should it sit with the fertilizer in it before using it?

I use 1/4 cup for a 5 gallon pot, mix it thoroughly, water it in and let it set for 3 weeks.. I use 4-4-4 but it should be the same..

Depleted, dry soil, may not have enough microbial life in it anymore. You may want to water in an aact, or add some ewc.