I don't think that the exact position, couple inches this way or that, is that important -- the proof is in the pudding; play and see which is most comfortable for you. The mistake a lot of pianists make is that they never experiment with different levels. For those who have, my few statistics indicate that their teachers almost always sit them too high (this means forearm parallel to ground). Mechanically, this seems best for most efficient transfer of vertical motion (tangent of circular motion of hand around elbow is perpendicular to keyboard). However, this also tends to force the pianist to play with the firgertips instead of the fleshy part opposite the nail. Thus the optimal position may be elbows slightly below level of keyboard. Bench height is adjusted so that this position is attained. The idea is to encourage the use of the fleshy part of the fingertip for sensitive playing and feeling the keys, and more use of FFP positions. These are the keys (sorry) to expression, pianissimo, and being able to adapt quickly to different pianos with different actions.
Some might object that it is easier to play FF by pushing down from above, but this small change in height makes no difference in FF; the real difference comes from whether you are playing FF from your shoulders, using the entire weight of your body (correct way), or just using your arms and smashing down (wrong way) which gives a nasty FF.