Yes, there are a lot of formulas out there. Some get so complicated as to make this extremely simple printing process complicated.
When I began salt printing a good many years ago I decided to research the original methods and that is what I use. Think of the technology of the era, and don't go beyond it. The chemistry is simpler and far less finicky. I use non-iodized sea salt, technical grade silver nitrate and insure that the paper is ultra light weight (20 lb or less) and gelatin sized. Beautiful deep browns and clear highlight result. I learned this from a hand written photographers printing diary from the 18th Century.
My only concession to the 20th Century is my HID UV exposure unit which provides consistency of exposure. If I need more contrast, I use my fluorescent UV box.