Okey dokey, first I think nobody should use an easy-to-memorize brainwallet for anything more than experimenting. I predict we'll start seeing very unhappy brainwallet users reporting huge losses sooner or later.
Second: the 'sentinel wallet' idea is all about incentives.
You need to put enough BTC in the sentinel wallet so it is economically rational for an attacker to "take the money and run" rather than spend time and money trying to crack a bigger brainwallet that might not even exist.
Again, I don't think you should use a brainwallet, so I'm not motivated to do the math to figure out how many BTC you should put into the sentinel wallet so a ration attacker will just take it, but that is the way you should think about it.