however the programming behind this i cannot get my head around.
For the second one, have a look at the map() command.
and im telling it that the values it should expect is between 0 and 5V but i would like it displayed as -40 to 125
Did you mean "the values it should expect is between 0 and 1023 " ?
I'm not sure what you mean by assign its units, but yep at display time is where I'd display the units, just stick a "%" or whatever on the end.Yep I'm sure you could use map to scale it from 0-100 as a %.