First, get rid of the copper! Unless you want your beer to taste as such. Use a barrier tubing and then restrict with 3/16" ID tubing. If you use approx. 50-60 ft. of this tubing and about 8 - 10 ft. of 3/16" ID vinyl, increase your pressure up to around 28 PSIG or so until you acquire the flow speed you desire.
At this pressure, any beer tapped overnight will over carbonate and be difficult the next day.
Scott Zuhse, Instructor
Micro Matic Dispense Institute