Hello all! I can use some confirmation on this because I do not use preservative often, heck I have only used them once before

I have Optiphen on hand and Soapbuddy suggested I add it to some liquid soap that for some reason would not thicken so I made into a buttermilk bath soap. Soapbuddy suggested adding a preservative to protect against the protein in the buttermilk. I know I am supposed to use 075% - 1.5% of Optiphen and I have 64 oz of buttermilk soap bath, so I am thinking I should add 0.64 oz of Optiphen? Thanks!