I don't think that you are going to get an good estimate of caloric burn. The ones I have seen have quite a bit of variation such as +-33%. It depends on so many variables. Obviously a lean cyclist in top condition will burn more calories say at 80% on the HRM than a person who is fat and sort of out of shape at the same intensity even though they weigh the same. Measuing C02 production is probably the best measure but I have never heard of anyone doing that outside of a laboratory.