I compared with Dutchpro and Canna pH down. Dutchpro shows "Concentrated – 30% Phosphoric Acid". CANNA shows "Corrosive elements, 59% phosphoric acid" and recomends "Add pH adjuster in very small doses (5ml) to the nutrient tank, checking the value of the pH level until desired pH level is achieved (6.2)".
https://www.onestopgrowshop.co.uk/products/dutch-pro-ph-down-phosphoric-acid?taxon_id=107
http://www.canna-uk.com/ph-series
Using 10 ml / l is reasonable in my opinion. But, Please add 1ml/l (or less) and check pH, and do it again till pH reaches the good range of pH. After you know how much pH down is needed to decrease 0.1pH, adding more than 1ml/l. I did also adding pH down 5ml/l first and adding pH up to adjust the pH value. It increased also EC and made me mad😨.