I have a similar homemade arch. I have a 350 cfm blower plumbed through my ash door. I retrofitted my ash door from a hinged door to sliding doors which split at the mid point. I welded an adaptor to recieve my air duct. When I first started using it, I found out that the blower was way too big. I was blowing my fire almost out, and I was wasting precious heat up my stack. I built a regulator to limit the intake on my blower so that it reduced the cfm. There is no doubt the evaporation rate was dramatically increased by the blower. My pan is a 2 X 3 and at peak efficiency I was showing about an 11 gph evaporation rate. I do have quite a bit of ash going out my stack. I have looked at ways to reduce this. Changes that I will try this season are installing a damper on my stack to act as a baffle and possibly screening the cap of my stack.