I'm trying to solve this and I'm not entirely confident as it has to do with dilution. This is all about trying to find the most cost effective way to obtain 70% isopropyl alcohol for cleaning.
I can buy a 32oz bottle of IPA at 70% for $2.15 ($0.67/oz) or 32oz bottle at 91% for $2.35 ($0.73/oz).
To obtain 32oz of 70% from a 91% solution then I need to solve for x:
0.91x = 0.7
x = 0.77
So 77% of 91% alcohol and 23% distilled water will provide a 70% solution.
So if the higher IPA is 77% by volume as 70% proof then 32oz of 91% IPA is equivalent to 39.36oz of 70% proof. And that comes out to $0.06 per oz.
Assuming that the distilled water to be added has no added cost, does my math check out?
I also believe that the final volume is actually smaller as the molecules of alcohol mixed with water is not 1:1 by volume. I think it's about 1:0.95 so the final solution would be more like 37.39oz... With the value being slightly higher.