It's a massively broad subject, as there are so many factors to consider when trying to predict future car prices.
We used Black Cap Book to value cars within a particular month and to be fair it also gives the drop in value per month, but only one month at a time as we dispose of the old books when the new one comes out.
For example: a 350Z 300 GT coupe 2006 06 plate with circa 30k on the clock lost £450 month on month. However the same car but an 2006 56 plate with 60k on the clock lost £850 month on month. An 2007 57 plate with 50k lost £950.
You can drive yourself mad with this, trust me!
The best you can do is use a much common sense with it as possible, by researching what’s out there and what it is realising. The Black Book is only a guide, the reality is, a car is only worth what’s it’s worth on that day to whoever wants to buy it at that price or bid price. The is especially true of specialist cars like the 350/370Z's.
when we value car to sell, we look at what is in the market place, and what its selling for. We then take an amount of money for general recon and a bit of profit on the forecourt. This figure is then sense check against the Black Book.
Good luck, but if you want to run any thougts by me send me a PM
T