Although impact decals can indicate the length of the club may not be right for the player, it is NOT a given that a toe mark means the club is too short and a heel mark means the club is too long. Those conditions of impact can be caused by several other things, including swing issues, swing weight, overall weight, etc. The goal as a fitter is to find the right combination of all the factors that allow the player to hit shots in the center of the face as consistently as possible for the level of ability they have at that moment in time. Also, as a player you have to be honest with yourself and and as a fitter you have to be honest with the player your fitting. If improvement through lessons would put them in a better position to improve their consistency on impact, you have to be willing to address it as a part of the overall fitting process.
Starting with a more obvious one: If the customer is consistently making impact on the toe and has otherwise good posture and swing technique, the club is too short. Start one inch longer and go to 1/2 inch over (half way in between) if he then consistently hits that one off the heel. Same process for consisyent heel impact. Check the lie angle as well since longer clubs need flatter lies than a shorter club for the same person.