amp gives good advice here. The knees should be tucked no more than one to two inches under the keybed. (I have long legs, so its 2" for me.) You should position yourself on the artist bench such that you are sitting ONLY on the front half of it, not farther back. You can adjust it first, as amp suggests, to a position where your elbows and forearms are level with the keys. That will mean that your level arms are about 10 1/2 inches up from the surface of the bench to start. Beyond that, it's personal preference.
I'm with amp in that I like to sit a little lower. I find that tone production is richer that way. But if you get too low, you'll lose dexterity in your playing. When you sit higher, the hands can become less flexible, so tone production becomes more dry and brittle in my opinion. The best thing to do is keep experimenting with it the bench. Interestingly, when my bench is at rock bottom low (which is where I keep it), I'm 11" below the keyboard. So going by the average 10 1/2" rule stated above, that means that I sit 1/2" lower than the norm. It works for me!