Talking, I'd say 'a thousandth' or 'a tenth of the %'. Using 'a' rather then '1' in indicating It appears nicer. More, in exposition, 'a' is surely an indefinite article, which indicates the speaker will not know the identification on the noun, In such cases the thousandth specifically. Auditors recommend https://andersonezqhw.creacionblog.com/36328478/the-2-minute-rule-for-thca-flower-pound