how will modifications affect my pattern?
 

"Shape shoulders: Cast off 12 (13-14) sts beg next 2 rows, then
12 (13-13) sts beg following 2 rows. Leave remaining 35 (35-37) sts on
a stitch holder."

So I had to modify my pattern slightly because when I went to measure
halfway through the back, it measured something bw 22-23" (when it was
supposed to measure 20.5") (weirdly enough, I went back and checked my
gauge and I was spot on... so perhaps it has to do with the yarn Im
using (bernat supervalue) when the pattern called for bernat
denimstyle, but anyhow...)

So I tore it out until the armhole and decided I would steek the excess
seam allowance (omg I can steek!!! I. CAN. STEEK. hehe) So I just made
the armhole deeper (pattern called for binding off 6 stitches, I did 10
(4 are going to disappear with the seam allowance, so Ill end up with
the original 6 after taking the SA into consieration).

Now my problem is when I'm supposed to shape for the shoulders... If I
cast off the required 12, then I won't end up with 35 stitches for the
neck. Would I cast off 12-4=8 instead?