No expertise on the bull as I literally got it yesterday and doing the burn in as I’m typing this. However, on my Yoder stick burner the upper shelf always cooked hotter than the grates...by a significant margin. I’m doing ribs and brisket tomorrow on the bull and will be using the upper shelf for ribs, hopefully someone will chime in.
Give it a little trial run if you have a multi probe thermometer. Place 1 on shelf where you will be placing it, and one on grate. Compare temps and figure from there.
If shelves same as 590, 1 rack per shelf