I think my top rack is about 7" from the top, giving me about 24" distance from coals. That said, I added a second grate about 6" below that one. My point is there's really no rules so do whatever works best for you. You can always add more bolts at a different height and leave the others so you have more options, just trim the bolts as needed for clearance when installing grates.
UDS'55&16, Akorn, Bar-B-Q Kettle, mini WSM, Drum Grill. I'm JD.