Some amount of fluid will drain back from the torque converter, which is unpredictable. Hence all methods are while running. Also, cycling through all gears is important to ensure all clutch and brake actuators are full. I used to work for a transmission manufacturer, so I am very familiar with the reasoning.

Drive vehicle for 20-30 minutes. Stop and cycle through all gears including S and L, stopping in each gear for 2-3 seconds. When you're back in park pull dipstick and wipe. Reinsert, pull again and check. If you need to add, do no more than a few oz at a time. Once you add, it gets a little tricky to check as the oil will obfuscate the reading. It is usually easier to drive a few more minutes to keep the temp up and then recheck.

If you follow this method you can get reliable readings. Don't overfill. Aerated fluid is bad and you'll get erratic shifting or flaring.