She is still lovely, even if she is shy
So, while I can't tell for sure, it does indeed look like she is an Exchequer leghorn. The primary differences (which are NOT percentage of white versus black, as leghorns get much more white as they age - called 'going grey'. If I remember correctly, Jubilee Orpingtons do this too...they get more spangled with each molt)
So, here are the primary 2 identifiable differences:
1) EL had rounder 'blobs' of white on black feathers, Ancona have V shaped white tips on black feathers
2)
Most Ancona have yellow legs with some black spots on them ( a few As don't, but it is the minority) EL have yellow legs, no black spots.