Had the same problem, found height needed to be higher, and move closer to the keyboard, absulutly necessary, just hold your arms out straight for a while and you'll see the back muscles up high are tight to augment balance. guessing a little back rest "like Gary's" also a plus.
I use a ultra bench with a 3" thick top, and its adjustable and folds up flat for moving.(I did reposition the mounts on the bottom to make it go even higher thought).
It lets me shift positions a bit, and I can sit for hours and it never bottoms out, Its very comfortable.
Also you should take a break each hour and move around, If you don't your also going to have circulation problems on the backside of you knees eventually. Everyone who works from a chair etc should very definately take hourly breaks......Pose
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