I’m wondering if both yellowlegs are Greater, on account of broader base to the slightly “upturned” bill which both appear to have, also the streaking on underparts is bold on both, should be more diffuse on Lesser?
Ratio of bill length to head 'length' markedly different, even allowing for heads turned at different angles, second image is about 1:1 whereas first is maybe 1.25:1. Does this constitute a diagnostic feature, or are the ranges of bill length sufficiently wide that this doesn't really help?
I never use it: not because it's not diagnostic - it may be if you can apply it accurately and consistently - but because it's pretty much impossible to apply it accurately and consistently. Fundamentally, you need to define precisely where bill stops and head begins: easy enough on a museum skin or a bird in the hand, but far from repeatable in the field or in a photo, and a small alteration in where you decide the dividing-point is will make a big difference to the ratio you calculate.
I commend just looking at lots of both species until you get a feel for the different bill lengths - it's the single best and most obvious feature - and, once you get it under your belt, it's surprisingly easy to make the differentiation. Lesser yellowlegs has a short-billed, straight-billed, dinky, elegant neatness which is distinctive and which greater yellowlegs does not have.