Field capacity is the point where it drains and stabilizes after saturation. Like when you do a pour through test, and saturate it, then let it stop draining, that is field capacity. It is pretty good advice. The thing about pots that is different from the earth is whats called 'perched water table' It's the effect of wicking, container shape, and soil structure the keeps the water content stable at a certain height in your pot. The higher and narrower your pot, the higher the water table. A lot of this depends on what is in your soil. The bigger the average particle size, the lower your pot water table. It's best to have a good mix of particle sizes, and well mixed so there don't end up being dry spots in the pot.