After updating my ATV4K to the latest OS Monday evening, the calibration process worked on my first try with an iPhone 12 and LG 2019 OLED in a darkened room. With my set up the calibration window on the TV was about chest height. I held the phone as close as possible to the TV without touching. Just a random data point. FWIW I am in the middle of the South Korean series Mr. Sunshine on Netflix. The series, depicting S. Korea in the early 1900s, bursts with colorful outdoor and indoor scenery as well as colorful costumes. IOW, a good test for calibration. After the calibration the character's skin tones seem to contain more texture, while the scenery, costumes appeared a bit "warmer".