I think I have mentioned this before.
If you are lucky enough to be able to adjust your machinery to the correct height for yourself, then the best starting point, and the one that works very well for me is...
Stand upright with your arms hanging at your sides. Then just bending elbows bring your hands up until they are 90degrees to the elbow. Get someone to measure the distance from floor to finger tips. That is your height to the table on the mill or bed on the lathe, without having to stoop or move your arms about too much. If you do use a seat for machining, buy yourself a good adjustable computer chair, so you can set it to the optimum height.
I buy leather covered ones that are dirt cheap nowadays, the reason for the leather, the hot chips don't burn it, easy to swipe the chips off when you sit down and no chips to get embedded in the fabric, that eventually get embebbed in a place on your body that is embarassing when you have to ask for help to get the bit out.
People will spend hundreds buying doodah's for their machines, but sit on orange boxes to do the machining. Comfort in your shop should be one of the highest priorities.
Feel comfortable, machine well.
This has stood me in good stead for many years.
John