This old inspection guide appears to be supported by all of FDA's example labels.
https://www.fda.gov/...s/ucm114098.htm
However the CFR was amended so that potassium doesn't fall with sodium anymore but with vitamins and minerals.
(iii) The percentages for vitamins and minerals shall be expressed to the nearest 2-percent increment up to and including the 10-percent level, the nearest 5-percent increment above 10 percent and up to and including the 50-percent level, and the nearest 10-percent increment above the 50-percent level. Quantitative amounts and percentages of vitamins and minerals present at less than 2 percent of the RDI are not required to be declared in nutrition labeling but may be declared by a zero or by the use of an asterisk (or other symbol) that refers to another asterisk (or symbol) that is placed at the bottom of the table and that is followed by the statement "Contains less than 2 percent of the Daily Value of this (these) nutrient (nutrients)" or "Contains <2 percent of the Daily Value of this (these) nutrient (nutrients)." Alternatively, except as provided for in paragraph (f) of this section, if vitamin D, calcium, iron, or potassium is present in amounts less than 2 percent of the RDI, label declaration of the nutrient(s) is not required if the statement "Not a significant source of--(listing the vitamins or minerals omitted)" is placed at the bottom of the table of nutrient values. Either statement shall be in the same type size as nutrients that are indented. The quantitative amounts of vitamins and minerals, excluding sodium, shall be the amount of the vitamin or mineral included in one serving of the product, using the units of measurement and the levels of significance given in paragraph ©(8)(iv) of this section, except that zeros following decimal points may be dropped, and additional levels of significance may be used when the number of decimal places indicated is not sufficient to express lower amounts (e.g., the RDI for zinc is given in whole milligrams, but the quantitative amount may be declared in tenths of a milligram).
c.8.iv refers to the table below that section, which defines the unit of measure for potassium to be milligrams and does not show more than two sig figs in any of the values.
Ergo, even though all the guidance floating around still refers to the sodium rounding rules for potassium, the actual CFR basically says that you can use whatever levels of significance you want on "vitamins and minerals".
Frankly, I think decimal points in nutrition facts panels tend to be red flags for regulators, they dont' look legit. I'd stick with the old rounding rules for the sake of not confusing some regulator who hasn't read the CFR like you have, and +/-5 mg of potassium isn't going to be significant in the scope of the RDI of 4700mg.
