First, we start with your height and weight and convert that to a rough BMI range. While BMI definitely isn't perfect as a measure of healthiness, especially for athletes or people with a lot of muscle, these ranges allow us to factor in body mass as part of our overall requiremest calculation.
Next, we use the activity level and goal you specifiy, along with the BMI range, to get a multiplier for protein that ranges between 1.2 and 1.6 grams of protein per kg of body weight. We multiply your weight times that multipier to get your recommended "Protein per Day". People who are sedentary will have a lower recommendation than people who are active, and people who are looking for the right amount of protein for muscle gain will have a higher recommendation than that of people who are looking for the right amount of protein for weight loss. We are using the "Optimal daily protein intake for adults" table on the Protein Intake page at Examine.com which has a great explanation with references to the relevant supporting studies.
At PlantFusion, we don't think you should get all your daily protein intake from just our delicious vegan protein powder, so we put a cap our on recommendations as "Daily Protein from Plant Protein". Our thinking is that you shouldn't have more than 3 protein shakes a day and our plant-protein shakes have about 21g of protein per shake. So, we recommend 50% of your calculated "Protein per day" or 3 shakes at 21g, whichever is lower. From that we make our "Protein Servings per Day/Week" recommendation.
Finally, if you tell us which of our plant-protein products you are interested in and how many shakes/day you would like, we do the math to find the best value subscription for you. It should get you as close as possible to the servings you need each month, so you don't end up with multiple bottles stacked up on the counter.