The "rule of thumb" about things to be hit with hammers is quite simple:
hit the nail on the thumb, that is.
For a serious answer, barring special circumstances, the nail has to go
all the way through the 1st piece of wood. This places a 'lower bound' on
the length. And you _don't_ want it to go all the way through the 2nd
piece -- because then the point would be sticking out. this places an
'upper bound' on the length. A "split the difference" approach works
just fine, between those limits.
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