I do understand that Cu(NH3)42+ forms a deep blue solution in water and hence the maximum absorbance would lie somewhere above 600nm but is there a specific lambda max value where the absorbance would be maximum?
The blue Cu(NH3)4(2+) according to the instrument Spectronic 20, is 610 nm.. I advice you to consult literature in order to match results with your conditions. Also use a perfect instrument to read it. Thank You and Good Luck