
 Assuming that when you say 50 units of N, P, K, you mean the equivalent to 50 lbs of N or P or K per acre.
Then, to get 50 lbs of N from a 555 product, you divide 50/.05 to get 1000 lbs of product per acre.
Give an acre is 43,560 ft sq'd then 1000/43,560 or 0.02295 lbs per sq ft.
If you need 0.0229 lbs of material that weighs 0.4 lbs per cup then .0029 divided by .4 or .0574 cups.
Given that 1 cup is equivalent to 48 teaspoons then .0574 cups is 2.75 teaspoons. (.0574 x 48)  
