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, Akorn, Weber Red Head Kettle, Drum Grill, Gasser w/cobwebs all over it. I'm JD.