This is just the observed variance. Which means that you assume that this will be the variance in the future.

Don’t make decisions for evolving systems based on statistics.

Insider info on the other hand works much better.

You can also trade bias against variance to minimize the mean squared error, which is the squared bias plus variance. This applies generally.

I wish there was a Strunk and White for mathematics.

While by no means confusing, it feels inelegant to setup a problem using variables A and B in the first paragraph and solve for X and Y in the second.