Mets are just another measurement of energy expenditure. Keep it simple. If you are doing cardio and it gets ez, bump it up a bit. If it gets a little too hard, ease up a little. Even if you didnt have any problems, your energy varies day to day. It also depends on your workouts. If you just did legs, obviously you are going to have a harder time with cardio.
I read a summary of a study of cardio effects on men and women and fat loss. To sum up: due to their differing metabolisms and muscle masses, women are best served by doing cardio for shorter but more intense periods. Men should go longer but less intense. So a "typical" woman may go 20 min at 80% hr and a "typical" man can go 45 min at 70%. In my experience, the harder you go the more calories and fat you burn. Sure, the % of fat may be lower, but you are in fact going to burn more fat overall due to the increased workload. For example, if you go slow you might burn 25 fat calories with a total of 100 calories burned. If you go fast, you might burn 50 fat calories with a total of 200 calories burned. These numbers are totally made up just to simplify the ratios. In the second example, you are burning more fat calories, even thought the percentage is the same.
The effects also last longer post training as well. That's just my opinion. Of course, I could be wrong.