Simplifying with memorable numbers 

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
200
(which have approximation errors of +2%, 4% and 8%).
For example, Earth's radius can be remembered as "Six ee Six
meters"
(6 × 10^{6} m, aka 6E6, which is
only 6% below the real 6.4 × 10^{6} 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 <mcharity@lcs.mit.edu>) 
In need of overhaul, integration with landmarks.