No problem. To control where the DoF starts, as in front to back: 1) in the Post Process tab; In the Depth of Field section (bottom), 2) Click the "Get Z1" button; Click on the image to set this. 2) Click the "Get Z2" button; Click on the image to set this. Z1 & Z2 are the controlling values for the depth. (It doesn't matter which is which.) It is often easier to set Z1 to zero & just set your Z2 val to where you want it to start blurring. You can also enter values manually too.
A couple of tries & you'll get it!
Another tip: Don't forget to click 'Reset' between tries as it won't automatically. But this can be useful as the effect accumulates - if you didn't get enough blur the first time, just click 'Calculate Now' again
If you meant: To control the intensity of the effect, lower the value in the "Aperture" box. Or raise it for more blur.