These are aspects which go into calculating your sitting position and stool height. The resulting figures may vary from person to person because of the length of the spine compared with the upper arm. Applying these principles will help you develop a good technique and reduce the possibility of RSI
Shoulders and upper arms of the pianist or keyboard player
Make sure that these are relaxed when sitting and playing the piano or keyboard. Standing to play the keyboard is not good for your technique, although many do this.
The shape of your spine when playing piano ...
should be that of an "S" not in the shape of a "C". The determines the height of your shoulders and therefore of the stool. It also reduces the risk of back ache and neck ache if you do many hours' practice. Don't practise longer than 30-40 minutes at a time and if possible do it in the morning. Remember that poor piano keyboard posture can cause RSI if you are not careful.
The shape of your hand when playing piano or keyboard ...
should use slightly curved fingers. This is very important for the correct hand and finger position and then positively active fingers will help you produce a good sound and develop finger dexterity and control.
Wrist position and arm angle
Make sure that you have a straight line from the inner elbow to the knuckles where the fingers start to curve. It's important not to drop the wrist because that makes it difficult for the passage of the thumb.
The angle of your arm - it's unwise to assume that we always approach the piano keyboard with the arm at 90 degrees. If the arm is more at an angle closer to 45%, then there is less work for the passage of the thumb - one of the hardest things to control in piano playing.
Piano stool height
Check the last horizontal, straight line - from the lower elbow to the surface of the keys - and this will tell you the height of your stool. If you like you can have the lower elbow a centimetre above the keys so that you feel your arm weight falling slightly onto the keys, but this is a refinement for later. It's not as difficult as it sounds if you have a friend (or camcorder) looking from your right side exactly level with the keys. A mirror will not work because of the angle.