First, whenever we work with an extract, the extract is our reference control, to compare its effect with the treatments.
The landa max depends on the type of extraction you perform and the type of molecule you expect to obtain. For example, if you do an extraction with a non-polar solvent, what you expect to extract is an extract with lipid compounds, and then based on the nature of these compounds is what you investigate or extrapolate the calculation you are looking for.
Good to see your answers, I want to know that, if someone work on extract/multicomplex compounds. May highest absorption peak of UV refer as standard lamda max of the extract.