Shank size has nothing to do with tubing diameter. The tubing found at hardware stores may not be "brewery approved" resulting in flavor and / or dispensing issues.
Is this "conduit" insulated on the outside and do you have any method of forcing cold air into this area from your refrigerator?
For Bud Light at 38 F, use 14 PSIG and again, start with six feet of 3/16" ID line and trim back in four inch increments until desired flow rate is acquired. After changing the line, pour two beers in a glass. Dump the first and take the temperature of the second with a
calibrated thermometer. This should be the temperature of the beer from the keg. Adjust to 38.
Scott Zuhse, Instructor
Micro Matic Dispense Institute