I recently called an airport operation manager and congratulated him for his 100% punctuality during the closure of his airport due to volcanic ash. In fact, canceled flights improve an airline's and airport's on-time performance though passengers are stranded causing additional cost and lost revenue. This is one of the reasons why I believe that punctuality is an imperfect measure, even though the aviation industry uses it as a benchmark.
I would suggest to use delay cost per passenger as a benchmark. The total delay cost is the sum of passenger goodwill loss (measured in money terms), cash out, lost revenue, unproductive resource usage (aircraft, gates, stands, etc.). Then put this into relation to the number of departing passengers and you eventually know where you are compared to your benchmark airports. And, you can use this metric to set targets and to evaluate investments into on-time performance.