The proper carbonation level of soda at 40˚F would be obtained with a pressure between 25-30 psi. You also need to balance the system with the proper length of 3/16" ID beverage tubing to get a proper pour. Just reducing the pressure to serve will work to some degree, but is a PITA, and if the pressure is not turned back up after each session, the volume of CO2 in solution will reach equilibrium with the new lower pressure in the head space and the soda will have less carbonation.
I always have a keg of root beer on tap for my boys. I have it at 28 psi and serve it through 20' of 3/16" ID beverage line, coiled in the back of my kegerator, to a perlick faucet. This set up achieves a perfect pour, no foam and a perfect carbonation level.
With regard to carbonating sugary soda, I have read, and found in my experience that sugary sodas are more resistant to accepting the CO2 into solution than beer is at the same pressure and temperature. My solution to this is to chill and carbonate the majority of the water first for a day or two. Plain water readily accepts CO2 into solution. I then heat up the remaining water to dissolve the sugars and extract in, cool it down overnight in the fridge, add it to the already carbonated water in the keg, mix it thoroughly and put it back on the gas at my serving pressure of 28 psi.