In your articles you say 2" of water. What is that reference to? 2" of water in a glass jar, quart container, gallon container, 5 gallon bucket?
If you can imagine your entire garden submerged under two inches of water, that's probably the most accurate way of looking at it. That's (estimated) how much your entire garden area needs per week.
Here's a page from The University of California's Gardening co-op Extension showing some visual references and a couple of formulas for raised and other types of gardens:
NC State University's co-op extension has some good tips on checking moisture levels manually: