The cone method you mentioned is by far the easier way to do this, unfortunately since I am using a school woodshop I don't have the luxury of being able to line up and glue and repeat. They won't allow glueing on their lathes, nor will they let you hold up the lathe for a long time unless it's for finishing.
I have to resort to measuring and making it as class to possible, then gluing. As of so far this method has been accurate to a point as to when I point it on the lathe there is little to no "wobble" from off center rings. But, looking forward to the day I can just glue up on the lathe!