I tune by using NGK plugs, -8 or -9 for 15/1. The plating will scorch off the base of the plug some as you get leaner, closer to optimum. I like to see about 3/4 of the base scorched, threads still shiny and unchanged. You will have to check all the plugs and eventually will see if you have one cylinder warmer than the rest. I go by that one for saftey. My #5 & 8 run a touch warm, so those are the two I check for tuneup, check the rest later to make sure all the rest are in line.