I always brine my turkeys overnight nowadays, usually from frozen so the brining is part of the thawing process. I don't use any nitite/nitrate cure for that at all but I certainly wouldn't put a tablespoon of saltpetre in a gallon of liquid
Having said that, when Mike Robinson dry cured venison for a couple of hours before BBQing it he seemed to sprinkle on pink salt quite liberally. I suppose the arguement is that not much will be absorbed if you are curing for a couple of hours only but in a brine a lot more might get in. I would stick with John's figure which is a happy medium between FDA and UK recommendations for an immersion cure or better yet don't use any unless you are doing a proper timed cure as opposed to a quick brining.
Dave