Here are a couple of sites to help with jet sizing.
http://www.robietherobot.com/NitrousJetCalculator.htm
http://www.dynotunenitrous.com/store/Scripts/Jet_calculator/nitrousjetcalc.asp
The n2o solenoid will want less pressure than the carb in almost every case with a reasonable jet spread. A regulator for the carb and separate regulator for your juice unit fuel solenoid is the best approach.
