ditto ditto ditto ..... to the earlier suggestions!!!
Perhaps enlarge your PP patterns to more than 100% until you get to the 12-1/2"?
Make sure that there is 1/4" seam allowances on the original, or add them.
Also, I always let my fabric go over the outside seam edges when doing the PPing.
Then trim to the perfect size after papers are off and it has been fully pressed.
Your asked whether it mattered if they line up on the outside edge?
Yes, of course ... you'll find there is a wonderful secondary pattern that will show, and if you don't have the matches, the flaws will show all the more.
You do stunning work ... don't let temptation let you down this time!