Thanks for that, I partly see where you are coming from with the factoring, but if the vane reads -7 degrees (indicating it is below the horizontal) and increases to -6 degrees its getting closer to the horizontal, giving less AOA at a slower speed. Shouldn't the reading decrease even further, beyond -7, to go further below the horizontal giving a greater AOA?!
As an aside I just re-read the info I was given and it pivots in the middle with the trailing edge being +ve when above the horizontal - this doesnt change anything just thought I'd correct myself!