-- Do the measurement first. Express the result in any unit you feel like.
-- If the number turns out to be inconveniently large or small using that unit, then convert the measurement to a different unit that produces a more convenient number.
Examples:
-- Measure the length of your kitchen, in miles. Result: My kitchen is 0.00227 mile long. Inconvenient number ! Convert it to millimeters. Result: My kitchen is 3,658 millimeters long. Inconvenient number ! Convert it to feet. My kitchen is 12 feet long. Convenient number ! Easy to say, write down, remember, and tell other people.
-- Measure the distance to the moon, in nanometers. The moon is 383,023,872,000,000,000 nanometers away from Earth. Inconvenient number ! Convert it to light-years. The moon is 0.0000000405 of a light year away from Earth. Inconvenient number ! Convert it to inches. The moon is 15,079,680,000 inches away from Earth. Inconvenient number ! Convert it to feet. The moon is 1,256,640,000 feet from Earth. Better, but still not too convenient. Convert it to miles. The moon is 238,000 miles away from Earth. Probably the best we can get. Still ugly, but we're running out of units. Try AU (Astronomical Unit, the average distance of the Earth from the Sun) The moon is 0.00256 AU away from Earth.
