I'm definitely not one to practice what I preach here (as I note below), but I've been told by PTs and docs that core muscle strength up front goes far to support what happens at your back. Not a chiseled 6-pack, but abdominal muscle groups being fit and firm do a lot for spine support. Then there is lower and upper back muscle fitness and lower body fitness, another subject altogether...
Lathe height- for 20-some years, my lathes have been at elbow height. But it depends where you measure that height. Tip of elbow with a loose hanging arm? Not for me.
With regular shop footwear on my feet, feet located below my hips, and hanging my arm to my side, I bend my lower arm up to 90°, parallel to the floor. Then I (or better yet a helper) measure from the floor to the inside bend of my elbow, right where to top surface of my forearm meets that 90° bend of my arm. (Or stand with your bent arm along a wall and use a pencil held level to make a tick mark of the height on the wall.) This height is somewhat higher (up to several inches depending on the beefcake build of your arms) than elbow tip of a relaxed hanging arm. But, this is the height limit for me- any higher, and although it may feel good to my lower back, tool control becomes sacrificed- wrists have to bend harder and shoulders hunch upward to maintain tool control. Not good.
Arm girth- if you're someone with big, muscular arms (I'd say I'm a fella of average build, with too much foam can cooler around an empty 6-pack!), you may want to temper the notion of going as high as I describe. Start there, but lower the lathe incrementally if the height doesn't work for those big guns of yours. Maybe start at the point of where the center of the elbow joint is located with the 90° bent arm, and go up or down from there. (All of this is just guesswork based on my personal experience. I'm no expert in body mechanics.)
There is no one-size-fits-all method to any of this. Your arms, from shoulder to wrist, may be longer or shorter. Your upper arm specifically may be longer or shorter, affecting the universal notion of elbow-to-spindle relationship. Get this measurement from both arms- maybe you naturally tilt to one side or the other, affecting the results. Only you will know what is right for your body, and that will take a bit of experimentation.