What was the resonance energy of benzene. And how it was calculated

The resonance energy of benzene is 36 kcal/mol.

 If we consider C6H6 as just a cyclohexatriene, the heat of hydrogenation should be 3 x 28.6 kcal/mol = 85.8 kcal/mol. However, when the heat of hydrogenation was experimentally determined for benzene, it was found to be 49.8 kcal/mol. Since hydrogenation of cyclohexatriene and benzene both lead to cyclohexane, reason for the difference in their heat of hydrogenation should be due to the difference in their stabilities. From this, it is clear that benzene is 36 kcal/mol (ie. 85.8-49.8 kcal/mol) more stable than ‘cyclohexatriene’. i.e. benzene with six delocalized π electrons is 36 kcal/mol more stable than ‘cyclohexatriene’ with six localized π electrons. Here, 36 kcal/mol is the resonance energy of benzene (Heat of hydrogenation is the quantity of heat released when one mole of an unsaturated compound is hydrogenated).