Using on-camera Flash Yongnuo 568EX II and Focus point and recompose question

I have a question with regards to using my new Yongnuo 568EX II Flash on my Canon T4i which has E-TTL II. I'm new to flash photography and am looking for some help.

I use my center focus point when I shoot and recompose my shot most of the time. I press the shutter half way to lock exposure/focus and recompose; If I use this method when using ETTL is the flash going to use my original subject exposure settings or is it going to recalculate with a preflash?

The reason for asking is that when I take a picture of 2 subjects standing apart I will put my center focus point on the face of one and recompose , that now puts my center point between the subjects pointing out into oblivion, I do not want my flash exposing for that. Does the ETTL II in my T4i allow me to use my center focus point, recompose and then still get the proper flash. Also, should I be using evaluative mode of spot metering mode.

Thanks

You should have no problem focusing and recomposing with E-TTL.

Spot vs evaluative may have a significant impact on the ambient exposure, but no impact on the flash exposure. The flash exposure is calculated just before the shutter opens by a weak pulse of light (the preflash) and is independent of the metering mode of the camera.

Have a look at Flash Exposure Lock in TOYUG, p.15 (link to pdf is in my signature.) Although the guide is about the YN-622C trigger, that section is pure Canon, and gives an over-view.

The simple answer - use the FEL button.