I'm not sure, but the way I read it was that the impulse is the slope of the first "vertical" bar of the N, rather than the slope of the diagonal line. So as the pulse travels, the vertical part gets compressed and steeper (I increases), and the height of the N gets smaller (Δp decreases), and the the distance between the bars gets longer (broadening).

But yeah, figure 17 on page 26 of the document you linked seems to be exactly what we are looking for, great find! One can see exactly how the waveform changes with altitude. Moving from 42,100 feet to 70,700 feet reduced Δp from 1.82 to 1.13, a factor of 0.71.