Biking for cardio - amount of calories burned?

lasooch

I know I won't get an exact result without a heart monitor (or probably even with one), but I need a good, solid guess. I'm cutting at 700 kcal deficit currently. I'd prefer the guess to be undershot a bit, but not too much so that I won't exceed 1000 kcal deficit. I want to be biking for 100-200 km/week, so a small overshot can make a big difference. I don't really trust myfitnesspal's calculator for exercise energy burned.

Now, I weigh 81 kg, I'm riding on a rather flat surface (not many climbs) at around 21-23 km/h average.

My current guess is 25 kcal per kilometer. Today I did a little over 28 km, so let's compare results:

My guess says 707.
Myfitnesspal says around 920.
Endomondo says 1369.

This leads me to believe my guess might be undershot a bit too much, but I don't really know.

Anyone with experience in the field?