There is no such thing as lead hairs.
Y'all would be telling me when my ends were getting thin that I had lead hairs when I never had them before until I stopped trimming.
That my hair used to be so full and then all of a sudden I had thin ends that stuck out like a sore thumb longer than the rest doesn't mean I had lead hairs. It means my hair strands were breaking off from wear and what I was seeing as "lead hairs" were the thin ends that were getting ready to break. Heck it was those "lead hairs" that kept me at SL for over 30 years of my life so I know what da hayle I'm talmbout.
The whole theory of "lead hairs" is an oxymoron coz what y'all telling yourselves is that out of 90% of strands growing, a large fraction will slow down growing so that some supersonic few spring forward and take the lead. And then these supersonic ones hit the brakes and wait for the others to catch up.
Nein, nein, nein! What happens is splits cause weakened strands to break off creating see-through hair. ALL HAIR continues to grow (meaning both broken off strands and the ones still holding on for dear life) and over time the see-through ends too meet their demise because they are weak from splitting and they break to where the others had broken off. So all hair is now the same length. But all the hair has been growing...so all hair now looks even and longer but not longer than where the lead hairs would be reaching given your growth rate. There was no catching up with lead hairs occuring. The so-called lead hairs broke off to the point where the other hairs were broken so they all look the same length now.
I mean, y'all can't really believe that if hair grows at 6 inches a year that LEAD HAIRS wait for the other hair to catch up? Why don't they continue to get longer and continue to be lead hairs? What possesses the follicles of those hairs to pause and wait? And what in the name of all that is sane made the follicles that hold these strands suddenly take off so they became lead hairs?
Makes no ounce of sense. But if y'all believe in fairy tales and they make your life rosy, then continue to live in delusion and to believe in "lead hairs".
OP, I'd cut at the blue line and then dust regularly after that. Trimming gradually when you have damage does no good but allows the damage to continue as it has been and retention continues to be a dream.