I think that the most common way to value Bitcoin is like valuing other currency by another currency.
Seeing to it that we all know all the factors that causes or affects supply and demand equilibrium.
Also the time we hold bitcoins. Time is very important factor.
As you've mentioned, bitcoins are valued by the law of supply and demand. But nowadays, since bitcoin hype was so strong a few years ago, a lot of people went all in on bitcoins and they've become whales. Whales are dumpers and pumpers of bitcoins in a huge scale. They can affect bitcoin pricing in a blink of an eye.