You bring up probably the most crucial point to any attempt to invest money for a profitable return which is money management.
I see no difference between the two terms , units vs. % of bankroll wagered. i understand why Thunder does his % thing as it allows him to adjust his amount bet according to his balance at any given time. The same can be done with units and it's a heck of a lot easier to track for most of us. The thing is, if you start with a hypothetical bankroll of 1K and your basic unit is $20 you would need to adjust your unit size as your bankroll increases or decreases over time. As an example, say your roll goes up to 1.5 K then you can increase your basic unit by another ten bucks. likewise if your roll goes down, you would need to decrease the value of a basic unit.
Start easy, I would recommend that your basic unit be somewhere between 2-4% of your BR. and if you really love a game go ahead and play 2 units for starters. Stay away from teasers, parlays and the other BS. Over time they will kill you. Do your own capping, trust your stuff and have some fun with a hobby that can be profitable as well as fun. Keep in mind though, 90% of the guys here lose money. They won't say it but it's true. They are the guys that post plays on every single football game that happens to be on TV and 10 games a day in hoops season. I break this rule twice a season, the 24 hour hoops marathon and bracketbuster weekend just to have a little fun. To win, you will need to stay sharply ahead of the curve. Use discipline and only wager on or against the teams that you really know.
I wish you well !