The way i see it midwar/big war separation is not about the packs ina clan,at many midwar clans u can find elite packs with big online , but about the Clans as a whole.
Big war clan =Full organised CPs with steady prime to participate at epics and events
Mid war =Clans that also have a big amount of solo players and players with out prime times, that can't have steady participation to events/epics etc .
The best way to solve this is i believe when a clan alliance dominated mid is moved str8 up to big war and if we end up with a server that is not easy to tell who is mid and who is big just remove mid content.