Even though your technique has some logic to it, it's actually not correct.
There is a sticky in the Betta Habitat section on how often you should clean your water. But all tanks should be cleaned once a week, tanks under 10 gallons, twice a week. Here's the info if you want to read it: http://www.bettafish.com/showthread.php?t=115758
Even my 180L tank had 30% weekly water changes.
I change 50-90% everyday, every second day if I'm lazy because my tanks are quite small and some are full of fish. I dont want ammonia and nitrite building up and making my fish sick
I don't change my water more than once a month any more, but I change out at least 80% when I do. It's been about 3 years since I stopped with the weekly water changes. My betta/frog tank doesn't get changed or a couple months some times. One would think my tank would have imploded by now...
Don't change your betta to often. For one gallon of water, change once a week, 2 gallons, every two weeks, 3 gallons, every 3 weeks & so on.
There are far too many factors not taken into account by this theory, such as filtration, tankmates, planting etc.
Don't be guided by rules of thumb. Be guided by ammonia, nitrite and nitrate readings from an accurate test kit.