There is also, usually, a discrepancy with regular rulers. Many start at the quarter inch mark making your measurement 1/4" shorter. What I like to do is get a wooden ruler, measure it to be exactly 12". I sand down and round the edges as I shorter it to 12". As I said, many rulers start at 1/4" so be sure to measure because a ruler can be off just like the
BathMate ruler.