green tea spray preservative?

hi! i was planning on making a green tea spray with a caffeinated green tea and mint, but i'm not sure what preservative to use. is optiphen generally good to use? how do you know how much to use?

the recipe i found is this - i'm new to diybeauty, so let me know if there's an issue with it! steep 5 teaspoons of green tea and 1 teaspoon of mint leaves in a cup of boiling water. let stand for 10 minutes. strain. cool completely. pour into bottle. can add essential oil - vitamin e (it didn't say how much to add, so i'm not sure what the best amount would be)

sorry about all the questions, any help is appreciated!