I'm assuming the 12.5" is the measurement before you assemble into rows?
So you'll be sewing 4 blocks into a row using 3 sashing strips, right?
Then you'll take the 3 rows and use 2 LONG sashing strips to make the top?
Okay, based on my assumptions above, you do need the nine (9) 12.5" vertical sashing strips and two (2) 54.5" strips (not 57.5" - think in terms of "finished" sizes and then add the last seam allowance, so 4 blocks * 12"/block + 3 sashings * 2"/sashing + .5" seam allowance = 48 + 6 + .5 = 54.5")
Altogether you need 6 WOF sashing strips. You can get 3 verticals from each WOF (totals 37.5") and then, as you said, will need 3 for the horizontal strips (piecing them either diagonally or straight). So 6 WOF strips * 2.5"/strip = 15". IF your fabric is cut perfectly straight and you NEVER make any cutting mistakes, then 15" is all you need. If you think you might need a 7th WOF strip to fix a cutting mistake or you think the fabric will need to be straightened, then get 18" or better yet 21".