Generally, if you place your arranger's action at pretty much the same height as a piano's, get a seat, stool or bench at piano seat height, you are sitting in what has been determined as the best, most ergonomic position for good comfort for at least a hundred years or more.

Either raising or lowering the position will tend to put your wrists in a less than relaxed angle, leading to quicker fatigue or reduced speed of motion. You can raise or lower BOTH together, and you will still be OK in this department, but you start to get into problems of the seat edge either failing to support your upper legs correctly or even worse, cutting into them and messing with circulation.

It's sort of hard to argue with a couple of centuries' experience, in this regard...