Nikon, at least on the D800, has a dedicated autofocus button, or a button you can assign this function to. It deals only with focussing, and once pressed, it fixes the focussing on whatever part of the scene you want, or whatever part of the scene your single focussing square covers. There's no need to keep your finger on the button (AF-On button, on the D800) as you move the camera for best composition.
The shutter button is then free to deal only with autoexposure and release of shutter. Such an arrangement allows you to select which part of the scene you want in sharp focus, or which part you think provides the best focal distance for maximum DoF, before devoting your attention to exposure.
With camera in manual mode, it then becomes straight forward to select which part of the scene you want to be given a full, ETTR exposure, by moving the focussing square over that part of the scene, say a cloud in the sky, whilst simultaneously half-depressing the shutter button and adjusting exposure with the wheel on the back of the camera as you look through the viewfinder, assuming you have the exposure indicator visible in the viewfinder, which indicates under/over or correct exposure.
Having fixed the desired exposure for the sky, such exposure does not change when you reposition the camera for the best composition, as long as you keep the shutter button half depressed.
However, whether or not the exposure indicator in the viewfinder is correct for an ETTR is another matter. I generally find that a modest overexposure, in accordance with the indicator, is required.
I notice there's a bit of confusion in that second last paragraph. If one is shooting in full manual mode then obviously there's no need to keep the shutter button half-depressed to maintain the manually adjusted exposure as one recomposes. It's in Aperture Priority mode where that feature is useful.
