LOL It's 60-95 percent.
!00% alcohol won't work, as you said, because it evaporates too quickly, and you need at least some water in there for the alcohol to even work (solvent and surface tension).
Numerous studies have shown that 60% alcohol will kill almost all bacteria, and viruses that do not have an external covering (an envelope). Coronaviruses do not have an envelope, so 60% will kill it. Alcohol does it by denaturing the proteins in the virus. meaning, it breaks down the proteins and they stick together (see, pan fried egg whites). More than 60% provides no measurable benefit.
So if you stick with 90% or better alcohol, and keep the mix as 2:1 alcohol to water, you'll be above 60%.
Technically, you should measure the alcohol content in the finished product to be sure, but how many people have an alcoholometer? Well, if you make beer, wine or moonshine, you probably have one. They're cheap. LIke, under $10 cheap.