Calorie burning

Some time ago (probably about a year) I was looking online to see if I could find some accurate equation which would enable me to calculate how many calories I expend whilst out cycling (the heart rate monitor I've got doesn't do this for me). Unfortunately I wasn't able to find anything.

I recall getting one (exceptionally long and complicated) formula which I didn't understand whatsoever and which didn't make any sense to me. One part in particular seemed like it couldn't be true. The equation said to add actual road speed and wind speed/resistance together to get one of the variables which made up part of the total. This being the case, it would mean adding (as an example) 15mph actual speed to 10mph of a headwind and coming up with a total of 25mph. If it was a 10mph tailwind and a road speed of 15mph, then it would total up to 5mph. To me, that's just bollocks. There's no way I (using the same numbers in the example) would put in the same effort to travel at 5mph into a 10mph headwind as I would (and this is how it would work out) to go at 25mph with a 10mph tailwind. It made the whole overlong convoluted equation completely pointless in my eyes. In fact, it made it so pointless you've got to wonder why I even mentioned it here.

I've looked at those websites where you stick in a little information (weight, length of time exercising etc.) and it gives you some general calorific calculations (eg. Fitness Partner's Activity Calculator) but would like to find some means of working it out more exactly. As most decent heart rate monitors are able to work out a total, surely there must be some (hopefully simple) equation programmed into them which they use to do it. If so, can anyone tell me what it is please and put an end to my fruitless search?

Something else I was thinking about is how much more effort it seems to require to increase one's speed a little. I'm not really concerned about how fast I go. I wouldn't need to be as I'm pretty slow but that's besides the point. When I was out with Kirby Girl and Gorsebank the other day, after having been out on a run alone the afternoon before, we went at (as usual) about three-quarters of the pace I normally go at by myself. I know I'm not making any sort of groundbreaking discovery here but going at a quarter less pace than normal is definitely more than a quarter less taxing. Presumably this is all related to wattage output and that sort of caper. Can anyone help with some means of working that out too? All help would be greatly appreciated, thanks.