The generally accepted guideline is the center height of the spindle should be at the center of your elbow. This is just a guideline, not a rule. Different people will find different heights comfortable. It will vary from person to person depending on a lot of factors. For me I find the general rule works fairly well however I just bought a bigger lathe and am turning more larger items. I am experimenting with different heights for the lathe. I started with it at normal height and turned for 2 weeks. Then I jacked it up and placed 2x6 under it. Turned for 2 more weeks. Then added 2x4 on top of the 2x6. I thought I was going to raise it a bit more because when hallowing bowls I was still bending over a bit but then I turned some spindles and thought any higher would be uncomfortable. I'm still experimenting.
Bottom line is experiment a little and what is the most comfortable for you is the right height. If your back hurts at the end of your turning day then the height may not be right.