Best settings for tracking a fast moving object with my T2i?

My 70-200 F4/L showed up in the mail yesterday. I grabbed the dogs and went to the park for a game of catch. I got some good pics with the lens, it's definitely nice and sharp but the majority of the pics didn't quite have the focus nailed.

I was shooting in Av f\4 (I probably should've been in Tv), AI Servo and using the center dot for AF area. I was keeping the shutter half pressed while trying to keep the dog in the middle dot.

Is this the best way to go about it? I'm just wondering if there's anything I'm missing.

It was at dusk and my dog is brindle so she probably wan't providing much contrast for the AF to pick up on.

I would move focus control to back button focus.

I read a little bit about that the other day on here but I don't totally understand it. Would you mind going into a little more detail or is there a link that would explain it well?

http://www.learn.usa.c​anon.com …ckbutton_af_art​icle.shtml

http://www.johnstuartc​larke.co.uk/back-button-focusing/

thanks, much appreciated

Images of fast moving objects from a T2i.

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