In the photo below I have temporarily installed some little red threads
to mark every 10 rows. I put them in as I go up the sweater. This way, I'm not endlessly counting and counting and counting the same rows over and over again. By knowing exactly how many rows you've worked for the Back...you can create a lengthwise mirror image for the Fronts.
I take the row markers out after the FRONTS and the BACK are all done, and of course, before blocking.
BTW: the white thread is marking the row in which I bound off for the armhole.