It appears that my confusion in Part I has to do with Mr. Ham's poor wording--and/or my inability to read properly--and not his math:
5% of those who are now 20-29 and at one point in life attended church regularly, stopped doing so before elementary/middle school. (Hence Ham's stated 95%)
40% of those who are now 20-29 and at one point in life attended church regularly, stopped doing so before high school. (Hence Ham's 55%... 100% - [5% + 40%])
Since 11% of those who are now 20-29 and at one point in life attended church regularly but now longer do so, were still attending church regularly during college. This complete shift in focus totally threw me for a loop. But, 55% - 11% = 44%, so 44% of those who are 20-29 and no longer attend church regularly, stopped before college.
Thus, based on Ham's wording, we must assume that this last 11% left during or sometime after college.
So, despite being incredibly unclear in my mind, his math does check out.
In Part 2, it turns out that I was the one who made the serious error. I rightly assumed that they should have been percentages, but my mistake was to assume that these two percentages from mutually-exclusive groups necessitated that adding them together should come to 100%. This was not the case. Rather, 34.3% of Group A agreed, 69.7% of Group B also agreed, 28.9% of Group C... and so on.
Thank you all for helping me figure this out! As frustrating as it is for me to learn that my math skillz truly are completely dull, I am happy to know that the numbers--when untangled from what I perceive as poor wording--do come out correctly.
As for Ham's conclusions, assumptions and ideas... well... I'll get to that at some point.
I am happy to report that the last chapter contained no math which confused me. ...mostly, I'm sure, because the last chapter had no math whatsoever. Had I been confused then, well, more than just my math skillz would be missing...
Thank you, again, to those who set me straight. I appreciate you taking the time to correct my thinking.
Filmmaker, Writer, Expectant Father