AFAIK only Romans had legions that followed the same pattern (of course with variation of allies and sometimes specially enlarged ones). So for the rest of the factions its a matter of

1. Looking at major battles of the period if they are around and work out ratios if you feel mathy
2. Think of something plausible that works in EB (AI turns as good for this)
3. Convince yourself its plausible and reflects what you've read or looked at(AI turns good for this as well)
4. Create plausible army

Rinse and repeat if something in environment changes and you can link it to a change in army composition