Hope this can clear up some of your questions. Regarding what is to lean. In an internal combustion engine what you have running on petroleum "complete combustion" of the fuel is approximately 14.7:1 air fuel ratio. Anything above this 15 ,16,or 17.0:1 is lean to very lean indicating a problem mechanically or by tuning. Ideally for an engine naturally aspirated you want (and this is a ballpark because every engine is different in design) to be somewhere at 13.8:1 at the beginning of your run and to finish at redline in the area of 12.5:1. For a forced inducted motor it will of course be a lot richer. For best results tuning on a "loaded" dynamometer where you can input your vehicles cd (coefficient drag) and frontal area for real world simulation is ideal. Mail order tunes are decent but they are based off of someone else's results that may or may not be exactly like yours, and are generally a lot cheaper in price than "actual tuning" on a dynamometer. For example I've seen 10 C32 AMG all make different horsepower on my dynamometer on the same day, and the highest made being completeley stock with 1,600 miles on it make 327 SAE corrected horsepower. So a tune for the guy making 299 rwhp may not be what's good for the guy making 327 rwhp. Alot is generally left on the table and in some cases to much can be done to the tune where it's actually a decrease in performance be it to much timing to little of fuel or the exact opposite.