Surprisingly, according to this website with an interactive map of the World of Ice and Fire, Littlefinger's "old flint tower" is not on the four most fingery Fingers at all, but here:
To find this image for yourself on the Quartermaester site: set your spoiler sensor to the end (or to as far as you've got, as long as it's further than the middle of aSoS2 when Littlefinger visits his ancestral home), select "Petyr Baelish" on the list of characters, and zoom in on the Fingers region.
As for canonicity, the Quartermaester map comes from here based on previous work from here. The first of these links has an official endorsement, though not any sort of guarantee of accuracy, from George RR Martin himself.