I don't know about where your at but most all bigger feed mfg.'s that I know use a software to figure a set batch of feed.
Also I believe your referring to the Parson's Square not Parson's Table and Sunflower Meal or Sunflower Expeller that is between 22% - 42% protein and not just plain Sunflowers (Black Oil Sunflower Seed or Striped Sunflower Seed) which has a protein amount of 15 to 18 percent.
The system that I use is simple, easy and used by some of the local mills that don't us a software plus you can use the same figuring for all relating to a set batch of feed (Proteins, Amino Acids, Fat, Energy, Vitamins, Minerals, etc..).
For example lets say your mixing 25 lbs. of Soybean meal and 75 lbs. Corn together and you want to know the ending protein amount.
(corn protein amount being 7.5% protein per 100 lbs. and the soybean meal protein amount being 48% protein per 100 lbs.)
7.5% protein ÷ 100 lbs. = 0.075% protein per pound
0.075% per pound protein x 75 lbs. = 5.625% proteins
Soybean Meal -
48% protein ÷ 100 lbs. = 0.48% protein per pound
0.48% per pound protein x 25 lbs. = 12% proteins
5.625% proteins + 12% protein = 17.625 Total proteins...