Don't even bother. Make all the blocks and then trim then down to the size of the smallest one. That way they'll join together easily.
Then, when you're done, take a look at the process again. The directions are correct, and should result in a 12.5" block.
But the blocks are off because something isn't right in the way you're cutting, sewing or pressing (or all or any combination of the three).
The log cabin block is very forgiving, and since those logs are wide anyway, it will not be noticeable.
You need the blocks all to be the same size more than you need enlarge them to come out to 12.5" (for this quilt).
Here's some more info:
http://www.quiltingboard.com/t-112724-1.htm#2925138
The part where I recommend Sally Collins' book, The Art of Machine Piecing. It's the bible.