Unfortunately there are no calculators for insulated smokers.
Best to get it in the ball park range with Dave O's reverse flow calculator (click on the reverse flow section).
If you could figure a way to insulate the heat exchange tubes would be good as that's where a bunch of heat loss will occur.
The smaller the tubes, the more friction you have.
2 x 3" tubes probably will be ok or even go to 4" on a 20 chicken sized cooker like yours.
yes a water tray will help just try everything you can so hot flames are not blasting onto your brisket.
i would make the chimney anywhere from 1/2-3/4 up the CC.
You want to get good airflow through the cooker