How to identify the wavelength to compare absorption at in spectrophotometry?

I boiled spinach solution (ground spinach leaves homogenized in 90% methanol) in solutions of varying pH, and am using spectrophotometry to determine chlorophyll content. My results show changes in lambda max, and all my trials seem to end up with varying wavelengths. I was planning on picking one wavelength to compare absorption between my samples, but due to these variances, I am unsure of how to choose a wavelength. How should I do this? I will attach examples in the changes of lambda max. (Lambda max is 403.0 nm in sample 1 and 410.5 nm in sample 2.)
Also, a community member suggested that I use a wavelength at which both chlorophyll a and b absorb light (where they overlap on the absorption spectrum), which is at around 450 nm and 650 nm. The peaks in my results do not match these wavelengths, but would it be okay to compare the absorption levels even if they are not peaks?
[![Sample 1: lambda max at 403.0 nm](https://i.stack.imgur.com/DT7tF.png)](https://i.stack.imgur.com/DT7tF.png)
[![Sample 2: lambda max at 410.5 nm](https://i.stack.imgur.com/PbbDN.png)](https://i.stack.imgur.com/PbbDN.png)


