That's hard.
Interesting though. For me, it's the opposite. From the point where I'm hanging with straight arms, up to the 'half-way' point, where my elbows are at 90 degrees - that's the most diffiuclt. If I can get there, the upper half is less challenging.
I saw a vid, where the suggestion was that one might start with your chin above the bar and lower to half-way, then backup again, to build strength in that section of the movement. I'll have to try it but for the lower section, perhaps.