1. What speed are you comfortable with? The faster the lathe speed the faster things happen.
2. Typically I don’t get too fussed about the actual speed but go to as fast as I feel comfortable with depending on the size and complexity of the item being turned. I then speed up until there is vibration in the lathe and back off until there is no vibration. Once you true the work piece, the faster the lathe will be able to go until you feel vibration.