I've been re-reading this thread to find some more exact dimensions on your setup in hopes of recreating them using my 3/4 Frosty T burner. I actually found some dimensions in a different thread. My apologies if you already detailed these but again, I did my best to search (including going post by post).
In one thread, you provided dimensions for your plenum as 2.5" x 8". Is 2.5" both the width and height?
You just previously posted that your holes are 1/8 (.125) and about .1675 (say 3/16) apart (good timing, thank you).
Do your holes begin about 1/2" from the edge of the plenum (so a 1/2" border of no holes)?
How thick is your refractory burner head? (I know there is discussion of going thinner)
How deep did you submerge your plenum into the refractory mold?