Shoot for having the tops of the seat even with the bottom of the window frame. There is no set rule as to how high they sit in the car however. That is determined by the most comfortable position you decide is best for you.
After looking at numerous junk yard seats and not finding anything that I liked, plus the thought of having them re-upholstered, I sprung for a Teas split bench seat. It was pricey, but the seat is something you will be spending a large amount of time in and it must be comfortable. I do not regret the money spent for a nice seat.