I also struggled with height - a good bench height to aid my alignment, height of wrist, palm, etc. This all became clear to me after I started studying the Taubman technique. There are so many variables with height. First of all, there cannot be one answer because we all have five fingers which are all different lengths. Realizing this made playing so much easier because I was not stretching or reaching anymore. I also agree that your height will vary depending on what you are playing - chords, octaves, ect. and where you are playing on the keyboard - are you in front of yourself, high register, low register.