What is important to realize about finding a comfortable sitting position at the piano is that the quality of the chair you sit on (as well as the distance from the piano) can drastically affect your experience either positively or negatively. MPR of not necessary, but relevant recently documented his quest to create the perfect adjustable piano chair, taking as his model the unique chair of Glenn Gould. MPR's description of his process is laid out step-by-step, so if you're interested in doing some chair mods yourself, you should be able to duplicate his results after a trip or two to the hardware store.
In their daily rounds, collaborative pianists often have a tough time of playing on many different pianos, each with a different quality chair. Variable keyboard heights (floor types and casters can affect this greatly) can make the winning seating equation even trickier to find. I know of many pianists who ask for a stacked second chair or phone book in order to sit higher. My problem is the opposite--since I'm so tall, I need to sit low, and can often be seen scouring the RCM for the lowest possible chair.