With reference to Ian's post suggesting possible total numbers for Whinchat in this area: I used to have a BBS in the area described which was highly populated by Whinchat in one part, so much so I used to check and double check that I wasn't over-counting. In 2011 I surveyed an adjacent valley (another side tributary of the Doethie) for the Atlas and could not believe that there was not a single sighting!
The difference seemed to be in habitat with the Whinchats preferring the valley with a more varied habitat, particularly bracken, mini vegetated ravines and a nearby small patch of woodland so I would be nervous of extrapolating up based on area alone as there are quite some differences in the habitats.