Target 38 F and if you are at sea level (1-2000'), set pressure at 14 degrees F. Duct work sound good other than use two inch inside of three inch or better yet one inch flex inside of two inch. You do not want to add any more additional cubic feet of space than necessary for the cooler to maintain. If the unit has to run harder to keep this additional area chilled, it will burn up prematurely and run colder than desired.
Route the beer line inside of smaller flex and try to insulate outside flex as much as possible with the blankets. You want to simulate your keg cooler insulation. stop the inch inch flex just short of the shank. Be certain that you seal the entry point of the beer line just past the blower (60 CFM may work better) and the air returns back through the large flex with no obstructions and no air leaks.
Use 13.5' of 1/4" ID with .5' of 3/16" choker. Less beer between the keg and the faucet, the better. You could also use 8' of 5/16" ID with 6' of choker to the faucet.
Scott Zuhse, Instructor
Micro Matic Dispense Institute