Just a thought that occurred to me as I was reading. ETTL (especially when set to evaluative) can get thrown off by a lot of things, but the camera specifically looks for the highlights in the scene to avoid blowing them out. That's why if you get a direct reflection of the flash off of something like a mirror in the background, your whole picture will drop down to a dark exposure based on the camera lowering the flash power. If you change the EC of the image, the highlights of an image may also shift once the flash is applied. For instance, if you're shooting someone in a white shirt against a light wall and you increase the exposure, the added flash power make the near-ground subject (person in white shirt) brighter in relation to the background wall. Whereas under the original setting, the background may have been the brightest area in the pic and the target for eTTL exposure, the new values make the person's shirt the highlight of the image and the eTTL calculations adjust accordingly. I would advise you to only use FEC when you're using eTTL. That will only affect the flash power applied and minimize the number of variables. I advise most people to only use one "auto" setting, which includes eTTL. Then you don't have to worry about the effects of each change on the calculations of another variable setting. So shoot in manual mode and if you want to change the flash ambient ratios, change your shutter speed or ISO.