mikeee12345
You can take a measurement from your good piece and use a compass to draw a template on a piece of cardboard or a piece of veneer that matches the diameter you want the sphere to be. You can hold the template up to the piece while turning and determine where you need to remove material from the billet.
The average person will never notice small variations in pieces that are a distance apart from each other.
Another option would be a duplicator setup on your lathe with a template setup to guide you on all of the dimensions for the spindle piece.