How do I know how much weight to use in different environments an in different wetsuits?

There are way too many variables at play in this conundrum for any single equation to work for every diver in every situation, so experimenting and keeping track of successful weights for specific applications in your dive log is the key to happiness. A good baseline to start with is 10 percent of your body weight (salt water, 3 mm wetsuit). For fresh water, subtract 3-4 pounds of weight. For thicker wetsuits (or drysuits) add a pound or two or weight incrementally.