proper Dilantin Level?

Hello!
7 years ago in Oct.2003 at age 49 I sufered a severe righ themisphere ischemic stroke. which left my left side apendages frozen. in the fall of 2007 I fell down and wound up sitting against a wall not knowing who I or my wife were or what happened , where I was etc. The EMTs and ER said I'd had a seizure so my primary care 'Doc' put me on 400 mg. of Dilantin a day, 200 at night/200 on waking.and hadn't had another seizure until sept 8, 2010.
My neurologist had lowered my Dilantin by 100 mg. a day over the last six months top try and improve short term memory problems it may have been causing. then while on this lowered dose I had a big Gran Mal on sept 8.at the ER this time they said my lvel was way too low and I was put back immediately to 400 mg./day contingent on a blood test after 1 week. The results of which I was only told that the Dilantin level is now 12 and right where the neurologist says it should be.
  so I'm wondering what is a Dilantin Level of '12'. and is it reasonable to think now that back to that level my seizures should again be controlled? And id 400mg./day a risky level to be on that drug forprobably indefinately?
    I was given an EEG 6 mo. ago which the neurologist said showed that I would have had 6 seizure 'spikes' during the test if he had taken me off completely. I certainly don't mind taking something that controls my seizures ( because being permanentlyearly retired disabled at home by the stroke I can't live without ability to drive around town) as long as it's not long term dangerous.