The main reason that your char cloth is uneven is that you didn't let it sit on the heat source long enough. As long as oxygen doesn’t get to the char cloth there is no way you can overcook it. Wait until the smoke stops coming out of the can before checking. Don’t overstack your char cloth pieces in the tin as it could also delay finishing your char cloth. It might even do it unevenly as the heat might not have penetrated all of the layers correctly. Regardless, just leave it on the fire and it’ll finish in time.