Measuring back of sweater question


I am knitting my first sweater (in garter stitch). It is Plymouth Yarn Company pattern #1431 Ladies Jacket. I have knit the back, increased to shape the sleeves as indicated in pattern. It then states piece should measure 12" from beg. Work even until piece measures 18". Bind off all stitches. My questions is…at 18" do i measure the “center” of the back or the sides which would include all the increases? Sorry if this is confusing! thank you.

The increases don’t affect the length, but you can measure up the middle of the back.

Thanks Suzeeq for the prompt response! Things are beginning to make sense now!