Southpark:
That other set of values you mentioned..... ie. [(0, 1.66, 3.33, 5)] ..... the 1.66 is actually 1.666666666666666666 etc. and the 3.33 is actually 3.33333333333333333 .... which isn't very good, because they just keep going, right? Maybe for a mathematical exercise it is all ok. But from a hardware implementation perspective, that's not what they do (or want to do), right?
This is not the point. The point is that if you know the Digital value is X what could be a likely original Analog voltage that generated that X?
What I am saying is if X=1 then 1.66 or 1.666666666666666 or 1.5 or 2 are as likely good guesses as 1.25 (in your example). the rest is litterature...