Subject: How do I calculate extinction coefficient of my product pNP

I am trying to measuring lipase activity(from fish) using substrate pNPP(p-
nitrophenyl palmitate) which would form pNP upon lipase react on it.  I am 
using spectrophotometer at 410nm to measure pNP formation. And I am not sure 
how do I calculate extinction coefficient of my substrate product pNP.  Do I 
measure the Absorbance of pNP at different concentration, and graph the 
result. Is the slope of the graph my pNP extinction coefficient? What is the 
unit then? Thank you!

Re: How do I calculate extinction coefficient of my product pNP