The first time I attempted a scrappy binding I went about 12” but not all exactly the same. Looked great until I started pinning to the quilt top.....I found I could not avoid having the joints end up at one or more corners. Because my lengths were irregular I couldn’t really just do the math. After an hour of adjusting I switched to a solid binding. My advice is cut your strip lengths based on the size of your quilt so you don’t have the same problem. For example if your quilt is 60”x80” you could use 20” strips if you start 10” from the corner. 15” strips would also work if you started 5” from a corner.
Apologies if I am pointing out the obvious but it was a real Doh! moment for me!