As a general rule, I do not recommend a high seat at the piano, because this induces the employment of the arm and shoulders rather than of the fingers, and is, of course, very harmful to the technique. As to the exact height of the seat, you will have to experiment for yourself and find out at which height you can play longest with the least fatigue.
The Height of the Piano Seat
Is my seat at the piano to be at the same height when I practise as when I play for people?
Yes! Height and distance (from the keyboard) of your chair—which should never have arms—you should decide for yourself and once for all time; for only then can you acquire a normal hand position, which, in its turn, is a condition sine qua non for the development of your technique. See also to it that both feet are in touch with their respective pedals so as to be in place when their action is required. If they stray away and you must grope for the pedals when you need them it will lead to a break in your concentration, and this will cause you to play less well than you really can. To let the feet stray from the pedals easily affects your entire position. It is a bad habit. Alas, that bad habits are so much easier acquired than good ones!
[3. POSITION OF THE HAND]
The Tilt of The Hand in Playing Scales
Should my hand in playing scales be tilted toward the thumb or toward the little finger? I find that in the scales with black keys it is much easier to play the latter way.
I quite share your opinion, and extend it also to the scales without black keys. I think the natural tendency of the hands is to lean toward the little finger, and as soon as you have passed the stage of preliminary training, as soon as you feel fairly certain that your fingers act evenly, you may yield to their natural tendency, especially when you strive more for speed than force; for speed does not suffer tension, while force craves it.
[4. POSITION OF THE FINGERS]
The Results Count, Not the Methods