[GingisK]

So how can you look at something and say, "That is exactly 300 cubits long", given that fact that a cubit is different for every person?

You can't. A cubit is a rough estimate. Nobody should be able to say "exactly" and "cubit" in a single sentence while keeping a straight face.

[Fiji Hill]

The great kilt, a garment made from eight yards of woolen fabric, was once worn in the mountains of Scotland and can be seen today at Renaissance festivals. Each pleat of the kilt is one cubit long—and it varies with each individual.