How to calculate the percentage purity using the HPLC?

I have run HPLC for my column purified D-gulucose gallate. In HPLC two peaks were analysed for alpha and beta isomers, I'm interested in calculating the percentage purity of my compound.
Any standard I need to use for the calculations? if yes, than which standard, and do I need to run it along with my sample?
How to perform this analysis?