Depends a lot on what you are turning, but standing normally with the arm bent at the elbow, the point of the elbow is the starting point. From there you adjust upward until you find the level that is most comfortable for you, your turning style, and what you turn. Spindles you tend to feel more comfortable at a lower level. Hollow forms up a bit from that. Bowls up a bit more. I have one of the highest lathes that I've ever seen even though I'm only 5'7". That's just my style though. Your milage will vary. Enjoy the process.
Working flat so I can play round,
Repentance Is The