How exactly do you think it was done for decades before digital?
In the electronic flash era, you use a flash meter and meter individual units for the ratio desired. In the flashbulb era, you simply adjusted the distance of the flash units according to inverse square law to get the desired exposure at the proper distance.
It is really not that complicated.
IMHO, TTL is great for a single on camera unit or even a single off camera unit, but TTL for multiple camera flashes is sort of snake oil. If it works at all, it is so complicated that is more hassle than simply doing it manually. Why? Because no TTL system can possibly read your mind and know what ratio you want between the various units. It can get a correct exposure on the film or sensor, but whether it can do what you have in mind aesthetically is questionable.