Brine ratios

Is there a set formula or ratio for a pickling brine? I'm fairly new to canning and pickling, but after reviewing several recipes, I've noticed that they tend to be allot more varied than I had anticipated. It seems to me that since the salt and acid in a brine are there to preserve the food, that there must be a basic ratio along the lines of parts acid/parts water/parts salt, that I would not want to dip below, for safety reasons.
Also maybe worth mentioning is that I am currently canning using the hot bath method. So if the ratio does exist, does remain the same for both hot bath and pressure canning, or would it vary according to technique?