I have honestly, and slightly embarrassingly, not tried to sex by bill color.  I read that it's about 75% reliable, presumably by pros, so figure in my noob factor and it would be about as reliable as a coin flip. 
 
One thing I did have good results with on my last (Jan 3-4) hatch was with assistance.  I didn't keep records (for a change LOL) but I think I helped 6-8 of 22.  My perception was they were having trouble with the membrane, like the hard shell portion would separate and chip away just fine, but the membrane wasn't penetrated.  Two I thought were goners for sure because I busted blood vessels but they all survived and I can't tell which I helped.  Can you?
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