There is quite a bit of radiant heat coming from the bp so it seems to me that at least 3" but some say more. It depends on you really because this topic really only applies to the bottom rack
Current Smokers: Backyard RF Offset and Hybrid RF Offset trailer rig with Cowboy cooker and fish fryer, always room for more........
My last build was only 2". I made the baffle 1" below center of tank along with the door cutout being 2". Made the bottom grate 1" above centerline. Worked out great for me all around. On my 17.5" tank I needed all the room I could so that I could Ft my 2nd rack.