Bounce flash Exposure

I understand that Ettl meters through the lense and sets the power of the flash for what it sees as the correct exposure . When using bounce flash I understand that the power of the flash is somewhat reduced depending on the height and reflectivity of the surface you are bouncing on. I also understand that the only way of adjusting the exposure with Ettl is is by using FEC.

Now I am taking a photo of a bride 3/4 length and the camera is set to Manual I am using a good bounce surface and and adequate Iso and aperture to give the flash enough kick to light the subject. Does the metering look at the white dress and try to expose it as grey 18% or because I am using manual it does not. If it does not in M does it does it do it in say AV mode.I know that bounce flash is a suck it and see approach and I obviously will be chimping and adjusting the FEC as needed.