Now I see your point.
A couple of hints:
If you want that your alternative plan works well for every resolutions and aspect ratios, you will unavoidably end up with similar to PDR method to calculate positions. I mean the offsets from center. That seems as an obvious and most reasonable technique for me.
To get clean exact map for af dots, what I usually do:
— Shoot something evenly lit (something flat and underexposed, or even with a cap on works fine for 650d)
— extract 50 to 100 frames with raw2dng
— open one of dngs in photivo and make adjustments: 1:1 view, turn off the green channel, use bayer pattern instead of any interpolation, output as 16 bit png, then export settings as a job file
— make the batch job to export all those frames as pngs
— open all pngs in photoshop as layers (document depth must be at least 16 bit )
— select all of the layers and convert them into the smart object
— go to Layer - Smart Objects - Stack Mode, and set something like mean or median. This gives you a better signal to noise ratio, thus ensures none of the random noise pixels will be identified as hot/dead pixel
— use curves to bring up the af dots and suppress the remaining noise.
From there it is not so hard to write down the positions in any coords system you like.
PDR does use exact positions, otherwise it just won't work.
If Set Dead Pixels option is enabled, it simply puts zero to every af dot. Clean zero interprets as dead pixel by every raw procwssor afaik. You can compare the interpolation algorithm by yourself, but my research gives me verdict that PDR does the best job so far. Just for example, PDR vs ACR: look closely to the edges treatment:
You can not just create a dark frame, because af dots intensities changes dynamically. The only way is to interpolate.
