I've been turning a bunch of 10" black walnut bowl blanks on my old Delta lathe. Back starts aching from bending over trying to see what I'm cutting in the bowl. My question is: Is there a rule of thumb on how high the lathe centers should be as I'm 6'. Maybe I need to raise my lathe up some? If so how much? (As I'm 76 maybe my back hurts just from age - ha.) Randy