The OP asked about mixing for use in an airbrush, for which this method works just fine without having to measure ratios.
Ok, I watched the video. And it's as I thought, he's mixed the entire bottle in one ratio, which makes the entire bottle useful for only one style of airbrushing-- and will change the paint chemistry in the bottle-- which most manufacturers don't recommend. Fine lines, mottling, elimination of overspray, and finely feathered edges may be more difficult to obtain with that single mix, not to mention the paint may become less useful over time. There are painless methods for mixing paint in proper ratios which don't affect the entire bottle. I personally like to use small clear plastic ml cups for mixing-- the type in which medicines are dispensed in hospitals. I pour in the paint I need to the appropriate ml marker, and add the amount of required thinner with a pipette, then just pour into the AB cup. These cups can be purchased through Micro-Mark or Model Expo a hundred at a pop- which means I can clean them or toss them if necessary. For Andy's one method, there are ten others on you-tube that recommend mixing Tamiya and other paints in separate containers.