Together with the EGT indicator you do indeed lean it off just until you detect a slight 'roughness' and then richen it up a 'couple of clicks'(depends on your richness control). Are you going to get the aircraft operating manual out each time you want to lean the mixture, to check the figure? With all the variables it is quite likely that your engine may not indeed be running at optimum richness even using this figure. Trust the tried and tested method and seek further advice from your instructor on the type.
PP