Although articles and calculators can give you a good place to start, trial and error is the only true way. There are a lot of individual metabolic differences that can make the numbers stated vary in terms of accuracy. I think 20 calories per pound of LBM is a good place to start. If you gain too much fat as a result of rapid weight gain, then tone down this number of calories. If you don't gain weight, then increase this number of calories. Make changes on the order of 5-10%, and wait 1-2 weeks before deciding whether a caloric sum works or not, as various factors can change your weight multiple pounds throughout the day (Hydration level being the prime factor).