I run the 20 tip jet burners with a 4" space (not adjustable) space and it works well. I reduce from 3/4" pipe to 1/2" and have never needed to run them wide open (15 gallon boil volume in 20 gallons pots). They are overkill for my system, but I designed it to be able to be easily upsized to 2x the batch size I'm doing now. I left the wind shrouds open a few inches in the front to make sure there was decent air circulation around the burner.
As for ASCO valves, pipe size is the thread size. Orifice size is the throughput. I'm using the Blichmann supplied 1/4" valves on mine (not on the boil kettle) and it works fine though not having to squish the line down to 1/4" from an already reduced to 1/2" line would be better. But mine is direct fired, so I'd be turning the valve down low on the MLT anyway. HLT gets my sparge water up to temp fine.