Wow, your times shock me, I have an 18.5 and have gone 16 hours with the water pan full and still have lump left, of course my ambient is higher than yours, but really, nearly twice as long?
Here is what I do different than you, I don't use a
chimney starter, instead I have cut both ends out of a #10 can and use a can opener and punch holes all around one end, I use a starter block and set it in the center of a full ring of charcoal, light it and place the #10 can over it and add 5-6 golf ball sized pieces of lump, when the lump is burning well both in the can and under it, about 15 minutes later, I remove the can and assemble the smoker. I start with all bottom vents open and after I assemble the smoker I turn the door upside down and hold it open about an inch and a half with a slotted piece of metal, this give me lots of air to 'turbo' my temps to 210°, then I close the door and set my bottom vents.
My lump is produced locally and it has pieces as large as grapefruit to golf ball size, I arrange the lump to get as much in as possible and as little space as possible between the larger pieces, I take the peas sized and smaller and pour them in the spaces between the larger pieces, this makes my charcoal ring quite full.
So..., maybe my longer burn times are due to, (1) not much lit charcoal to begin with, (2) large pieces of lump, (3) packing (arranging) the lump in my charcoal ring.
I hope this helps someone.
Gene