One way to simplify a number is to find a nearby one which is easier to remember.
For example, 218 might be more easily remembered as 222, or as 210, or as
(which have approximation errors of +2%, -4% and -8%).
For example, Earth's radius can be remembered as "Six ee Six
(6 × 106 m, aka 6E6, which is only 6% below the real 6.4 × 106 m).
Different people find different things easy to remember. You just try on possibilities until you find one which is both easy enough and close enough. It usually takes a couple of tries, and sometimes it doesn't work.
Finding numbers you can remember is important when you are creating your own landmarks.
Comments encouraged. - Mitchell N Charity <email@example.com>)
In need of overhaul, integration with landmarks.