hi wolf -
add 7/8" for precision cutting -
12 1/2 + 7/8 = 13 3/8"
easier - add 1" to unfinished size
12 1/2 + 1 = 13 1/2
then cut down to precise 12 1/2" blocks -
you will really only lose a sliver.
for grid cutting - the formula is
unfinished size X 2 + 1
12 1/2 X 2 + 1 = 26" squares - --> eight 12 1/2" HSTs
this formula will yield slightly larger blocks that you
will have to trim.
hth
lefty in philly