The paper refers to ethanol, and not isopropyl alchohol which is a larger molecule and also toxic to humans. There are also differences in their effects on living tissue which is why isoproply alcohol is used as a skin preparation rather then ethanol.
It may therefore have different effects on the roots from ethanol.
Since the authors of the paper ( http://horttech.ashspublications.org/content/16/2/294.full.pdf) have only tested ethanol, and do not know the mechanism of action, then you're conducting a different experiment.
It's unfortunate that their paper uses the word alcohol in the title.
As for the math, if you dilute it 50% 4 times, you'll reach a final concentration of 4.35% which is close enough to their working range, and easy to do for experimentation.
Edit: I found another website that also claims isopropyl alcohol worked just as well. It's unclear if the claim is just being repeated, or if it's based on experience.
Tippling Tips for Paperwhites . . .
To get a 5% solution from a 40% distilled spirit like gin, vodka, whiskey, rum, or tequila, add 1 part of the booze to 7 parts of water – this is an 8-fold dilution, yielding 5% alcohol.
Rubbing alcohol (isopropyl alcohol) works just as well. It is usually 70% alcohol when purchased, so use 1 part rubbing alcohol to 10 or 11 parts water.
Do not use beer or wine, as the sugars in them will cause problems.
As with humans, paperwhites can suffer alcohol overdoses. Do not give them more than 10% alcohol.
It's thought that water stress is differentially affecting stem and leaf growth, but does not affect the flowers.