Maybe it's handy for the British branch of the inferial nightmare but it's not exactly simplistic for metric.

Say I want the tap drill for a 10 x 1.5 mm thread. I line up "metric tapping drill" with 10 as is shown in the photograph. The TPI now reads 16.9. How many machinists can do the math needed to convert that to pitch expressed in mm per thread? Well, when you do that you get 1.503... which is close enough. You look at the outer ring and see you need a 0.323" drill. Now, if you're using metric drills you need to convert that to mm since nothing shows on the dial in the millimeter ring. So, you quickly mentally calculate 0.323 x 25.4 and get 8.2 mm which is close to the right value but will produce more than 75% DOT making threading more difficult?

Of course, you could have done the whole thing in your mind with a trivial calculation...

10 - 1.5 = 8.5

Now, maybe someone can explain how you use the chart to find the tap drill for a 10 x 1.25 mm thread.

Another puzzle...The chart shows 8 mm "clearing drill" lining up with 0.323". Does that imply that 8 mm = 0.323" (presumably for small values of 0.323)?