Paul, just to be a critic, which is always fun here, I'll say your illustration provides nothing useful because it only goes half-way. To determine whether a change of the front from 50 to 53 makes a bigger change than going from 12 to 11 on the cogs, we need to pick the bigger of the two ratios (53/50; 12/11). Just saying 12/11 produces a 9% change doesn't answer the question really. Looking at both ratios, it's obvious that 12/11 > 53/50, so changing the cassette will produce the bigger increase in gearing.
Actually, what I normally would do is just look at the two gear ratios, 50/11 (4.54) and 53/12, or 4.41. By taking the ratio of the two, 4.54/41, you can see that the 50/11 gear combination is 3% bigger (or "faster").
But for some people, the concepts of arithmetic and ratios aren't all that easy. For them, I don't see why we need to toss out the old 3-to-1 rule of thumb. It's rough, but answers the question.