When you carbonate beer you need to use the actual beer temperature to determine the head pressure required to hit your target. The easiest way to do this in a small keg is to measure the skin temperature of your keg somewhere in the middle. This will get you really close to the average temperature of the contents.
Things are pretty simple in a small environment with minimal variation in temperature from top to bottom. Larger tanks have a larger temperature variation from top to bottom and the hydrostatic head at the bottom of a tall tank can be a real factor since 33 feet (10 m) of liquid represents 15 pounds of pressure. This is a hairier problem that you fortunately are not wrestling!