One thing that can be confusing is not knowing how the percentage is calculated. There are two methods, percentage by volume (the volume measurement of additive compared to the total volume of solution) or percentage by weight (the weight in Grams of the additive compared to the total weight of solution).

For example, the volume of 5 ml of sodium added to 100 mls of solution results in a 5% solution, but the weight of that amount of sodium, 2.17, in the same amount of solution with a specific gravity of 1 g/ml, is not 5%, it's 2.17% (I think, it's a very long time since I did these calculations on a daily basis)