Where does the green part of the scallion start and the white part end?
If you want to be really, really picky, then the 'green' is each individual leaf as it breaks away from the cylindrical whole.
That leaves you with the decision as to whether to pick off each leaf, giving you a bit of extra 'white' or just make some relatively arbitrary decision after you hit that first leaf 'node'.
So, unless you want to pick off each leaf, then it's 'about here… ish… '

Personally, if I'm being 'a bit picky' I will unwrap the outer layer for the first couple of nodes, where they go unattractively lumpy, purely for aesthetic reasons &amp; include the rest in my 'white' distinction. Once I get far enough I [that I can't be bothered any further] then I pick out any remaining 'white' bits &amp; go all-in for the rest  of the greens, until I hit the raggy ends, which join the roots in the compost bag.
Once you've actually cut to the green line, separation for 'fussy distinction' reasons does get easier, as you're no longer dealing with the entirety of the onion &amp; its reluctance to part layers without ripping down to the root - you can just pull bits off &amp; deal with them separately.