measuring the back of a sweater pattern?

I am knitting my first sweater - pattern is all garter stitch. I have knit the back and shaped the sleeves, increasing as indicted in pattern.

The pattern says "piece should measure about 12" from beg. Work even until piece measures 18". Bind off all sts. Here is my question....when measuring do i measure from the first row to what is on the needle (the middle of the circular needle) or to the very tip (along the outside of sweater back) I'm sorry if this is confusing.... thank you for your help.

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