Tilt/Shift and exposure
Moving the lens only affects the exposure meter, not the exposure. The meter gets confused by the light coming from angles it's not designed for.
So if you need to meter, do it before making any lens movements. Of course this can be a bit awkward, since you may spend a fair bit of time adjusting the lens' movements, and in that time, the light can change, for example if you're outdoors.
Just use the meter to get yourself into the general exposure range you need, and make a test exposure. View the exposure with the histogram, as you state, and then adjust your exposure accordingly. Pay no attention to the image on the LCD in adjusting exposure; only use the histogram. And, of course, after making the first test exposure, pay no more attention to the meter, unless there's a dramatic change in the lighting conditions and you need to get back in the ballpark, so to speak.
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