^Good video. First for awareness of mislabeled products, second because I love that method of calculating laser wavelength.
It is very precise, typically limited most by uncertainty in the spacing of the diffraction grating. Or slit width if you happen to use the single/double slit method.
One other thing to be aware of about different wavelength and power lasers is that the relationship between apparent
brightness of the beam/dot and its true intensity
is very deceptive. Eye sensitivity to light varies by wavelength: peaking at ~550nm (green) and dropping quickly in the red and violet. My 120mW violet laser looks about as bright as a typical <5mW green laser, but will quickly sting bare skin. Definitely not