99% to 70% Alcohol

Standard 70% rubbing alcohol is in short supply right now ... I most recently checked Walgreens, CVS, and Wal-Mart, and all were out.  So I stopped in at the distillery down the street from me as they are making hand sanitizer (like many are right now) and I thought I'd buy some (didn't need it, but thought I should make sure I have some).  

While chatting, I told them that 70% isopropyl is in short supply and they volunteered that they buy 99% isopropyl as part of their production process ... so they sold me a gallon.  But 99% pure isn't all that valuable "as is" as it doesn't kill bacteria very well (lots of articles about why, so I won't go into detail).  

So I wanted to reduce it to 70% and re-fill the 3 partial bottles I have of 70%.  So the formula (without getting into all the math) is:

4 parts water, then 9.7 parts 99% isopropyl gives you 70% isopropyl.  If you want easy math, then:
4 parts water, then 10 parts 99% isopropyl gives you 70.7% isopropyl ... but weaker is probably best, so ...
4 parts water, then 9 parts 99% isopropyl gives you 68.5% isopropyl

And yet another way ... you want a quart of 70% isopropyl, then it's 9.34 ounces of water and 22.65 ounces of 99% gives you a quart of 70% isopropyl.