The FDA released a final rule: https://www.gpo.gov/.../2016-11865.pdf
says:
(6) A product that is packaged and sold individually that contains less than 200 percent of the applicable reference amount must be considered to be a single-serving container, and the entire content of the product must be labeled as one serving. In addition to providing a column within the Nutrition Facts label that lists the quantitative amounts and percent Daily Values per serving, for a product that is packaged and sold individually that contains more than 150 percent and less than 200 percent of the applicable reference amount, the Nutrition Facts label may voluntarily provide, to the left of the column that provides nutrition information per container (i.e., per serving), an additional column that lists the quantitative amounts and percent Daily Values per common household measure that most closely approximates the reference amount.
Basically, if your product is more than 1.5 servings (RACC) but LESS than 3 servings (RACC) then you label it as 1 serving. In your case that means labeling it per 50g. You may choose if you wish (I wouldn't) to go dual column and list both per 28g and 50g on the nutritional panel.