Hey Smokin Jo, here is what I come up with.
Feldon's has a few glitches when it comes to RF's The increased FB to CC opening size will result in much better heat, smoke and air flow. These figures come from trial and error.
My son and I build 1 to 2 pits a year during his slow time. These calculations work better than Feldon's .
Also: 33% of the CC for the FB is really the minimum you can take it on up to 35 -40 %.