Use Empty object as a Focus to better control DOF. Just place it where you want to have sharp spot and it's done.
Matching Viewport Preview with Render:
To match both views you need to use F-stop values for both Aperture and Viewport also you need to check High Quality. And lastly turn on Depth Of Field under Shading tab (N to open/close Properties Region).
Check your Focal Length and F-stop Values as they are crucial for DOF. Lower F-stop value will give you shallow DOF (more blur), higher Focal Length will also give you more blur but it will distort perspective.