First thing is to adjust the seat so that your eyes are in the correct place - line up the sighting balls.
Then you need to be able to apply full rudder and/or full toe brake in each direction.
Then adjust the arm support so your hand can comfortably hold the side-stick while your forearm is fully supported.
Finally, adjust backrest and lumbar to be comfortable. Some aircraft have knackered seats, in which case bring your own lumbar cushion. The lumbar region is just above your hips and should push into the curve of your back there.
In the cruise, you can afford to move the seat back, since you will be unlikely to need to apply full rudder etc.