Is there a minimum distance the grid should be above the diffuser plate ?
Would 1 inch be OK ?
I'm sorry I leaped before I looked , if you are asking about the Horizontal Smoker in your Pics, then I would say about 2" to 3" above so the radiant heat doesn't interfere. You want a roll of the smoke and the convections will do that , but too close and the heat has no room to diffuse.
Also, use the NO-PEEKY technique , if you are looking , you are not cooking - you retain a lot of moisture also... just sayin'....
Have fun and...