Stefek Zaba wrote:

The best aide-memoire I've come across (and therafter remembered!) for
such "cached" intermediate results is "pi seconds is a nanocentury" -

For pi itself everybody knows 22/7, but far fewer can recall 355/113 -
remember it as 11-33-55, split in the middle with the first half as
denominator - and that gets you within 0.1 ppm or 6 decimal places of
the real true pie.

Then there was Grace Hopper's visual-aid-memoi in some of her talks
she handed out 'nanoseconds' - one-foot lengths of wire, [...]

Funny, before I even got as far as that paragraph I was thinking "one
foot per nanosecond"...

For something slightly more on-topic, how about 60 bricks per square
metre (for half-brick thick stretcher bond)? Or ten ft^2 in a m^2 -
that's about 7% out but still useful for rough estimating or heat loss
calculations.

--
Andy