What salmonmac said and you'll see how the stitches look in your yarn. That looks like a fun knit. If you end up doing squares you can mattress stitch or other seaming technique or you could pick up and knit on the sides of two squares and then do a 3-needle bind off for your seam. Someone posted a picture of a blanket (?) they'd made and used the bind off for the seams but I don't remember who it was.
I should never overestimate my basic arithmetic skills.