I think it depends on what profit margin your after. I ignore all the guides most of the time.
I see what I would retail the car for and work a margin in.
I agree some cars make crazy money, I was up Aston Barclay today, managed 3, one had a blown headgasket, great!!
Sometimes cars cap quite low and are way out sometimes. we would all be way better off if we could buy cap on most.