Mits had a scale with a height gauge that we used to measure height. Two other guys there were also shorter than they claimed. It is likely that Arnold was 6-2 in the morning but lost half an inch during the day. I remember that he was trying to be as tall as possible when I measured him. I measured Arnold while others are guessing when they see him standing with others. Camera angle can make a big difference. The guy in front or closer to the camera will look taller.
Kurt Marnul measured Arnold's height several times at different ages to be 6 ft 1 or 185.5 cm
So your measurement seems correct, especially as you state Arnold made himself as tall as possible, so perhaps that resultet in half an inch extra.
Anold now probably is 180 cm or 5 ft 11 due to age and loads of wear and tear because of decades of heavy lifting.