There is an overall 2–3 kcal/mol error using the Benson group-increment theory to calculate the ΔH. The value of each group is estimated on the base of the average ΔΔH° shown above and there will be a dispersion around the average ΔΔH°. Also, it can only be as accurate as the experimental accuracy. That's the derivation of the error, and there is nearly no way to make it more accurate.