Coke yield on incoming feed is roughly 30% by weight, depending on the feed properties and operating conditions of your unit. This means that the vapor rate out of the drum is 70% by weight of the drum feed. The feed at the drum inlet is roughly 20% vaporized, so 50% by weight of the feed to the drum is generated within the coke drum due to the thermal decomposition reactions.
If you used 50% by weight of the normal drum feed rate as the vapor rate from the drum during a power failure case that has to be off-set by vaporizing liquid in the fractionator on an enthalpy balance basis, I think you would be in a defensible position for sizing relief valves. You could adjust for your feed slate and operating conditions and use the delta vapor generation on a normal basis as the vapor generated by the stored energy in the coke drum during a power failure.