There's calculators (and maybe still some formulae) on the web. There's calorie estimators available on GPS and heart rate monitors. I didn't know about the power meter option (but I'm not surprised).
Your body weight is the ultimate integrator. It might be fooled for a few days by fluid loss or gain, but if you're staying the same weight, you're eating enough over a week.
Just a guess, but you might want to go for a pure carb snack in the half hour after you finish your long ride. The idea is to reload the glycogen in your muscles, and there's a "window" of about 30 minutes after you finish exercising when blood sugar will magically replenish depleted muscle sugar (glycogen). After that it may take a couple of days to reload. And go back to your eating well habit after you've enjoyed that snack!