In practice, printed percentages are usually right. The error a recheck most often turns up is not arithmetic but units, and it is worth knowing what one looks like, because it is invisible unless you divide.
A microgram (mcg) is one thousandth of a milligram (mg). On a panel the two are set in the same type at the same size, three characters apart, and almost nothing about the layout draws attention to which is which.
So the tell is a percentage that cannot be reconciled with its own amount. If a line declares a figure in micrograms and claims a full Daily Value that is itself stated in milligrams, one of the two is wrong by a factor of a thousand — and from the panel alone a reader cannot tell which.
What to do about it: look at the other lines. If every other nutrient on the panel sits at or near a full Daily Value, the likeliest reading is that the unit is a typographical error and the percentage is right. That is a judgement rather than a fact, and the honest way to publish it is as a discrepancy with a view attached, not as a finding.
And then check the direction of the risk. For most nutrients a thousand-fold ambiguity at label amounts is harmless in both readings: too little does nothing, and the correct amount is an ordinary Daily Value. It is only worth alarm if the upper reading would approach a tolerable upper intake level, and those are published too.
None of this can be done here, and it is worth being exact about why. There is no unit on this product to be wrong, because there is no amount to carry one. Sit with that for a moment: a label with a thousand-fold unit error on it would still be more informative than this one, because an error can be spotted, argued about and corrected, while an absence cannot be checked at all. The three sums this desk could run on Breathizen are the bottle life — 60 mL ÷ 0.60 mL a day is 100 days, not thirty; the cost per day, $0.89, $0.59 and $0.49 across the three packs; and the rating, where the supplied distribution gives 4.34 against the 4.7 printed while the ten published reviews average 4.10. Not one of those three is about what is in the bottle.