1) Anything you eat while riding wearing a proper kit automatically equals zero calories.
2) Food or drink breaks get you a 50% discount on any calories.
3) Post ride (within 2hrs) gets you a 25% discount on any calories. Beer, of course, in any quantity within 2hrs of finishing a ride gets you 100%. Therefore any beer consumed could safely be assumed to be zero-cal. But seriously - here is a very easy formula --
Take the average speed and multiply by your total weight (lbs) (you and your bike) , then multiply that times 60 to get cals / hr.
So for example --- Arakawa River Train for me :
My Weight - 155
My Bike - 19
Total Weight -174 + 1 lb for shoes = 175lb.
So - I just kinda keep this 'magic number' in my head of 1500cals /hr. Of course going up a hill increases the rate and going down decreases it. And speed / intensity change it as well. So - for me personally - I figure I'm burning no less than 1000cals/hr and probably no more than 1700/hr. Then I just keep kinda mental tabs on what I'm eating. So far its worked pretty good - I've lost 17kg this year (stable).
Burning down the visceral fat is the hardest (and what beer, unfortunately, adds on quickest) This requires working out in the fat burn zone for at least (it seems) 2hrs minimum for 3x a week to see any results. I got most of my weight loss struggling with this and ended up just doing either Arakawa up / down or multiple Palace laps every other morning for several months. Boring - but pretty effective. Rain or shine. I'll start this again from next month to hammer off the last 2-3kg of gut remaining... ugh!
But there are many factors involved in calorie expenditure, not least of which is how fit you are. A cycling heart-rate monitor (HRM) programmed correctly with your weight, age etc. and with a fitness test function might give you an accurate reading. Or it might be wildly inaccurate because your individual metabolism may not be 'average'.