What is minimum sap depth that you can have on a 20" by 6" deep by 4' long Evaporator pan?( wood fired 55 gallon drum with 6" flue) it has 2" ceramic blanket on bottom and sides and fire bricks in lower portion. I also added an old fire place wood grate ontop of lower firebricks to give better air flow for burning.i insulated it to not burn my whole woodpile up!!!i hope!thanks for insite in advance guys.