Please read the attach publication (JM Betz et al.). According to the publication,  different  contents were found, if the determinations were performed using individual specific standards. and normalization using one standard only. See Fig. 1 and p 47. In this case you want to determine phenol using  Gallic acid. In my personal opinion, it will be nicer if used identically standard (= phenol) instead of Gallic acid (different compound).

Thanks for the suggestions and the article!

After you draw the standard curve for gallic acid prepare three dilutions for your plant extract in the same manner you follow for gallic acid and measure their absorbance at the same wave length then calculate the equivalence from the equation

please see this file

Thanks Fatima!