For typical TN, the reason many nueros perscribe it first because it is actually a diagnostic agent as it works oftentimes.
I vary my dosage as pain varies. Usually in the 400 mg range but oftentimes I dose up to the 800mg level. Even at 800 mg, I've had times where the pain bled through. At those times a 1000 - 1200 mg level would often times affect my vision, amongst all the other nasty side affects this "dummy pill" delved out.
I speak of typical TN - mine is V3 (left jaw) for 20 years. I've had gamma, MVD, and rhizotomy. I still run low dosage of tegretol (200-400) daily because, well, just because.
I have tried other drugs folks speak of. My drug of choice for pain relief remains tegretol.