I used to get 60-65 gph boiling rate on my old 2x6 with good wood, filling the firebox about 10” under the pans. This was an airtight arch with AUF and raised flue back pan with a rear hood and preheater. Try to keep your stack temp at least 900 degrees. On my current evaporator, I average about 1200-1300 degrees using mostly pine slabs. Even if it’s a standard arch you should be at least around 25-30 gph.