ABTS is best and reliable method for Laccase activity at 420nm, ABTS concentration of the mixture should be 0.2 mM. Then you measure absorbance over time at 420 nm. The concentration that you use is good (0.4 mM) but you can increase until 1 mM if you want to screen the little activity. The enzymatic units (U) defined as the amount of enzyme transforming 1 µmol of substrate per minute of your enzyme stock solution per liter are then calculated by the following formula:U/L = (∆E×Vt)/(ε×d×Vs) with ∆E being the change in extinction of light [min-1] at 420 nm, ε being the molar absorption coefficient of ABTS [M-1 cm-1] (coefficient is pH dependent), d being the layer thickness [cm] in your cell that the light has to pass, Vt is the total volume you measured and Vs is the volume of the enzyme stock solution you added to the ABTS stock solution.