The only one that could possibly be accurate is one where you actually are professionally tested while performing your activity, then having someone slap a heart rate monitor and a spirometer in your mouth to measure oxygen output, while they stick a noseclip over your nose.
Other than that, I really don't think a calculator would be able to accurately predict what amount of calories you burn, mainly because it doesn't take into account your level of conditioning and/or the amount of lean mass you have. Since muscle weighs more than fat, if you have more muscle than the standard (that they use for their baseline comparisons), they could put your calories burned as lower than what you're actually burning. The opposite is true if you have a low muscle mass- your BMR could be calculated higher than it actually is.
There may be a testing facility in your area, but I can't remember offhand. I'd check with the local universities in your area and see if anyone needs test subjects for a research project that may be performing the tests so you can have them done for free. Barring that, find out if they charge for the general population to come in and get tested, and if they don't perform the tests, see if they can recommend someone who can.
Once you get those test results, you will know EXACTLY how many calories you burn while cycling, and how many of those calories burned come from carbs, and how many of those calories burned come from fat.