How to determine % transmittance wavelength for establishing an specification of a compound and at what concentration.
Please run spectrum of the solution of analyte compound on the spectrophotometer and find out the wavelength at which there is maximum absorption (Lambda max.) taken place. From this absorbance find out E1% 1cm (Absorbance) of that compound.
Select appropriate concentration of the compound in that solution based on initial run and E1% 1cm value so that you get adequate absorbance (or transmittance) on the spectrophotometer (generally between 0.300 to 0.750) at the wavelength of maximum absorption (Lambda max). Absorbance above 1.00 is not preferred.