CV is added to the knob position as if the Knob were a 0-5V signal.
So with the knob all the way down a positive 0-5V CV input should result in values roughly 0 to 1 (due to offset error it may not fully reach both ends).
With the knob all the way up a negative 0-5V CV input should result in values roughly 1 to 0.
With the knob at noon, 2.5V should reach around 1.0, and -2.5V should reach around 0.0.
Hope that answers your question!