Afaik phenobarbital is lethal, but not the same like "N" (=Pentobarbital). If u use tablets they might have a "mechanism" so you cannot OD on them but I'm not sure. However LD50 doses (source Wikipedia) that means 50% of experimental animals died at that dose. For an OD attempt - assuming you react the same like those animals - you would need to double that amount:
162 mg/kg (LD50, Ratte, oral)
88 mg/kg (LD50, Maus, i.p.)
185 mg/kg (LD50, Kaninchen, i.v.)