Brad already posted that it's "Nodes above white flower", and it's a valuable measurement from bloom to maturity.

During early and peak bloom, you want it to be 7-9, and see it taper off to finish out the plant. We have some stressed dryland stuff around here [I imagine you and the rest of the cotton belt have the same thing here and there] that is "bloomed out the top" and short, where total nodes may be 16 to 18 and NAWF 1 or 2.

When NAWF hits a consistent 6 or so and we are in late bloom, it's slowing down right. When it's 5, it's "cutting out", and whenever the inner node spacing on the top 5 nodes is tightening up, and we don't need to worry about Pix.

Using Pix effectively is part art and part science, sometimes "shoot from the hip" works pretty well.

"NACB" is nodes above cracked boll, and that's one I really look forward to counting! That is when we gauging defoliation readiness, meaning harvest is coming and the season is coming to a close.