40D Back Focus: Little Help!

This will be a no-brainer for you, but I've used my entire brain and can't figure it out. So, I want to try shooting an event using the back focus function (and AI Servo) on the 40D. I can focus with the back button, very cool, but when I snap the pic, the press of the front shutter button seems to insist on checking the focus again, when I want to be able to freeze the focus using the back focus button so I can recompose, etc. So, is there some setting that will allow back focus only? I want the shutter button to take the pic with no focus delay...that split second can make all the difference. Thanks!

Take focusing off the shutter release button by setting C.Fn. IV-1 to option 3. Focusing will be done by the AF-On button only.

Custom Function IV.1 will allow you to remove focusing from the shutter button.

Thanks, guys! No experience whatsoever with those custom settings, but it appears I'm about to get a crash course. Appreciate the wealthy of knowledge everyone exhibits, and so generously shares, here.