The best way to convert your absorbance measurements in mole concentration is to calibrate your measurements with kwown concentration samples according to the well-known Beer Lambert's formula : A = eps * L*C where eps is the molar absorption coefficient (L.mol-1.cm-1), L the lenght of your cell (cm) and C the mole concentration (mol.L-1). This is a typical principle use in spectrophotometry analysis in liquid.
In order to perform this calculation you need a linear curve like the Beer Lambert formula. That is you would have mass on your x-axis which in your case is mM Hydrogen Peroxide, and the y-axis is Absorbance. Since pure Hydrogen Peroxide does not exist you will have to dilute your x-axis standard to the expected mM range of H2O2 using the labelled concentration (~30% is common) and the density of your standard.