Jesus H. Christ.
These solutions mostly seem to follow my logic in OP except for the known problem of volume shrinkage and some methods take that small difference into account. But since you're not mixing pure alchol with pure water, I doubt the difference is significant in this operation.
In other words, I don't think precise percentages are necessary. MillCreek cited a study a while ago which found that 70% alcohol (I believe isopropyl) was more effective in killing some germs than 90%. Hence the concern about dilution. However, I doubt the germs could tell the difference between 68.47% and 72.92% alcohol concentration.
So unless my friend is embarking on a quantitatively significant project I'm going to recommend the methods outlined in my OP but note that the amount of additional water should be reduced slightly*. If he desires more precision, I will provide MillCreek's link in Reply #4.
Incidentally, I finally found an old bottle of 91% iso, but the label did not specify whether that percentage was taken volumetrically or gravimetrically. They do provide a "Questions?" phone number, 1-800-632-6900, and if I ever get curiouser about this, I might call them. For myself, I think the gravimetric method is more supportable in terms of eliminating that shrinkage factor question, but volumetric is OK as long as you report it that way with details on how the shrinkage factor was dealt with.
I found Nick1911's "3.2%" remark amusing. :)
Terry, 230RN
* I'll never forget seeing a recipe for something years ago whose instructions included adding a certain ingredient: "Enough to fit on the point of a knife."