He's right, to find a constant, you simply calculate the relationship between two things. We do it all the time, but the most valid example I can think of is money.

1$ is worth four quarters, so how many quarters is 10$? The constant is 1:4... it's a really simple thing. Now, if you didn't know how much the value of a quarter was, you'd experiment. In this example, by attempting to purchase something, and comparing how many quarters you need as opposed to how many dollars.

Now, admittedly, when your dealing with atomic and astronomical calculations, things get a LOT more complex, but once you finally have the numbers (no small achievement), it's really easy to switch it back and fourth, simply by applying the constant to the value.