You can take the value of standard from a lab fellow who might have run the standard in the past or look for the value of standard in published literature and use that.

Find the attached of this article, it will be useful for you.

U can use standard curve from the literature...but it's better to bring the standard and run the experiment for samples and standard together

We don't reuse the same curve of any standard, only if you work in the same conditions in your lab (the same spectrophotometer, etc). Try for example to take different curves from papers and compare , you'll see the difference. In your case, you can take the standard curve from your lab member or someone who done it before and used the same apparatus than you for your flavonoids.

thanks for all of you

Benabdallah has made a crucial point.