I don't think 200 is enough at all. Atleast 500 to get a picture in my opinion. But still it's only like 75% reliable cause you can run into swings so probably anything around 1000-1500 will give a clear picture. But then again there are swings that take 1000 matches alone. You just have to be lucky my friend. That's all.

agree with Levian
 
200 is such a small samplesize. Even at 1000 matches it is difficult to calculate your ROI.
e.g.
Let´s say you play 1000 matches ($10 + 0.5)
You win 525 of them, you are break even
You win 550 of them, your ROI is about 5%
You win 580 of them, your ROI is about 10%