I put the bottom rack in the center of the cooker so that it's as wide as it could be and I keep the throat as low as possible on the cook chamber. I know that didn't give an exact measurement but it varies depending on the size of the CC, don't forget to leave a grease dam at the throat to keep the grease out of the FB.
I read different things about this, but my recent one ended up having the top of throat about 2” underneath the bottom grate. It was a football shape, so only top was that close. Also, my grate was at mid level so I could get the full 24” pipe depth with the grate. And I did position firebox so I got about a 1-1/2” or so grease dam
BigT is right about location. Square FB helps keep the shape of the throat opening under the grate the best. And square CC and square FB is even better still. The football shape is caused from both parts being tubular so it lends to a higher throat opening just for the sake of getting the right area of opening.
Making memories!
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........