You are doing the check for a lean or rich mixture by reading the spark plugs.

Another way would be to monitor the O2 sensor output while driving with a DVM. I would have to look it up, but I think that it is 451 Millivolts that indicates correct stoichiometric ratio. More voltage than that indicates a rich mixture and less indicates a lean mixture. The voltage vs. fuel/air ratio relationship is EXTREMELY none linear.

Good luck,