Depends on how you live, USD 800 in black market will fetch you something like ETB 20,000.
Lets say you rent a villa house somewhere in Ayat region which is outskirts of town, rent will be ETB 10,000 (minimum) you left with ETB 10,000 , now if you are planning to travel across Ethiopia and again you won't be eating local food from day one (as it will take time for you to accustom to local flavor) I seriously don't think ETB 10,000 is enough.
Now if you choose to live in condominium (studio apartment)or service of some villa then your rent would be something like ETB 4-5 K, in that case I think for single person ETB 16-15 K is more then enough to live in Addis Ababa.
This all is valid if you are travelling alone, with family USD 800 is strict no.