Also the price ranges won't be big enough - I looked up the price range for a Ford Ka vs a Ford Mondeo and it is nearly 4x between the cheapest Ka and the most expensive Mondeo.
So on the plus side making them each 5% isn't so far off. On the negative side it doesn't really work. Hmmm, back to the drawing board. Or is this why Marc didn't include such a factor in the Core book?!
I was actually thinking of multiplying the percentages so +1 +1 +1 -1 would be 1.05 x 1.05 x 1.05 x (1 / 1.05) =1.1 and +5 +5 +5 +5 would be (1.05)^5 + (1.05)^5 + (1.05)^5 + (1.05)^5 = 2.67.
-5 -5 -5 -5 would be (1 / 1.05)^5 + (1 / 1.05)^5 + (1 / 1.05)^5 + (1 / 1.05)^5 = 0.367
Those don't seem enough to me, so try 10%, which gives 1.21, 6.77 and 0.148 as as the multipliers.
[(1.05)^5 meaning 1.05 x 1.05 x 1.05 x 1.05 x 1.05 in case the notation is unfamiliar.]
Possibly the minus side could be a different percentage to the plus but that would mean that the baseline couldn't be chosen arbitrarily - if you have cheap, mid-range and top-end cars then you want to generate the same prices by setting the cheap price and having the values as 0, +1, +2 or by setting the mid-range and having -1, 0, +1 and that means plus and minus need the same ratio
Phil Kitching