Since you said that the agency she works at takes 15% of her 3% commission it looks like what happened was...
From the 400k 3% goes to the Realtor so $12,000
From that $12,000 15% goes to the Broker so now she only has $10,200
From that $10,200 she is giving you 66.66% which is around 6,800
And she is keeping 3,400
(You could also calculate it by taking the $8,000 which was 2% of the real estate agents commission and take 15% away from that - gives the same number $6,800)
Since I don't have the exact numbers that are my best assumption, but that is not a big sum that the realtor is getting. Unless this was a deal that was very quick or the agent was bad/didn't do a lot of work, I would not see this a very fair for the agent. That amount of money is similar to small lease/rent deals, not deals with buying 400k homes. So I would be grateful to the agent regardless.