If you weren't riding too hard, the 308 sounds rightish. I mean, it's all approximation anyway, so if you're close you're close. I know for light effort cycling I burn something like 10 kcal/minute and with moderate effort closer to 12-13 kcal, but I'm a big guy (215 pounds), so your expenditure should be a bit lower. If you were really pushing very hard the whole time, I could see the polar being right, but even at my weight it's hard for me to burn 15 kcal/minute.