I did the calculation when I accepted the deal and offered the specific btc amount (200), and then he tried to recalculate to that day because the exchange rate was up.
After doing my best to guilt trip him he gave me what I had asked for in my email to him. (Almost.)

You will get totally screwed (rather than mostly screwed.) if you accept your principal in usd at the time of your deposit with bitcoin as high as it is now.