Please find attached the calculations I use for my lactic acid. I have already added it in the additives section, and used the SAP function to account for the added sodium hydroxide that should be added.
As for the percentage, those calculations are for 100% lactic acid, so you can divide the result by the purity percentage and you obtain the amount that should be weighed.
(i.e.: if you need 1 gram of 100% lactic acid then for a purity of 80% you need 1/0.8 = 1.25 of lactic acid at 80%.)
Should you have any questions or comments, I am always ready