Surprises like why am I getting a value in femtovolts, minus infinity or NaN. And 32-bits fixed point is perfectly all right. Go something like 4.28 and you get 0-16V with precision in the tens of nanovolts range. Or whatever range you know you need.
Now that you mention - the range won't exceed the supplies available. I think mostly these particular voltages go through op-amps on the +-15V rails, so signed 5.27 or perhaps 8.24 would be plenty of both range and precision.