To the OP, what species of Cod did you smoke. I find rock cod as being way too dry to use in a dry brine. The Ling cod we catch is fattier than rock but I still use a wet brine for best results.
For our salmon, I use the basic 4+ to 1 ratio of dark brown sugar over non iodized salt. The size of the pieces dictate how long the brine period will be. All my fish is smoked at low temps, 125, 135, 145+.