I got ready to measure flatness of my bench, and realized that I don't
know exactly what is meant by the various measures I have seen.

I've seen statements about a table's "flatness", expressed in
thousandths of an inch or in thousandths of an inch per foot, e.g flat
to .002" or flat to .002"/ft. Are people using those interchangeably,
or does the first mean literally over the entire surface, deviation
from one point to another is no more than .002? And in the second
measure, does that mean that using a 1-ft straight-edge, any dip in
the middle is less than .001" and any rise in the middle will cause
one end of the straightedge to be high by .002" when the other edge is
held down?

In a wooden bench, I am much more worried about eliminating twist than
absolute flatness, but I wonder what is a reasonable goal? I figure
that if I can't fit a .005 feeler gauge anywhere under my 4-ft
straight-edge no matter how it is oriented, that I have attained more
flatness than I can reasonably expect to keep for any length of time
due to normal wood movement. Is that right, or should I strive for
better?

This is my first experience flattening with hand planes, and I am
pleasantly surprised at how easy it is to get pretty darned good
flatness with just a home-made scrub, a flea-market #5c, and my prized
"worker" 100-year-old #8.
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