The idea that the sound wave speed should be roughly equal does not hold water if you are looking to quantify deflection because, as shown by the US Forest Products Lab testing, the speed of sound propagation through Maple can vary considerably. Unless you use this actual speed, along with mass, to compute what deflection is, then the answer is not going to be correct. It is also going to vary with shafts from the same maker so a number obtained in one shaft does not necessarily mean that all shafts measure the same and unless certain parameters in wood preparation are met than the numbers absolutely will vary considerably.
Also, all high (LD) deflection shafts are not built the same. The point(s) that the shaft is bending from would also make a difference as while the last 4" of the shaft might be extremely light the area directly behind that might be extremely stiff.
Once again, the testing done by Barioni, shown here on AZB and ridiculed by most, came to somewhat different conclusions than what is thought to be 'common knowledge'.
I guess I should have defined what I mean by "roughly"...
rough·ly
ˈrəflē
adverb
2. in a manner lacking refinement and precision.
synonyms: approximately, about, round/around about, around, circa, in the region of, something like, in the order of, or so, or thereabouts, more or less, give or take; nearly, close to, approaching
Although the speed of sound propagation through maple may "vary considerably", that variation, especially among maple that has been turned into a cue shaft, still fits within my use of the word "roughly" when I was implying that the variation in the distance the wave propagates down the shaft during impact would be on the order of a couple inches. If you wanted to accurately characterize a particular shaft, I agree that you would have to precisely measure that speed. In your example of a very light area followed by a very stiff area, the stiffness would factor into the wave propagation speed (and likely the mass) and thus the effective endmass.
The point is that the part of the shaft that affects squirt is limited to some distance from the tip, so anything done to the cue beyond that point is not a factor.