Explanation of Apdex calculation doesn't make sense

[tdb-alcorn]

https://github.com/prometheus/docs/blob/main/content/docs/practices/histograms.md?plain=1#L93

The docs currently say:

Note that we divide the sum of both buckets. The reason is that the histogram buckets are cumulative. The le="0.3" bucket is also contained in the le="1.2" bucket; dividing it by 2 corrects for that.

This doesn't make sense - dividing by 2 has nothing to do with whether the histogram buckets are cumulative. Rather it looks like the 1/2 factor is meant as the weight of "tolerable" instances in the Apdex score. To correct for the fact that the le=1.2 bucket contains the le=0.3 bucket, you need to instead subtract the latter from the former.