I've made several quilts like this but marking the center is a good idea. Myself I prefer to use a foundation fabric like old ugly stuff or muslin.
A couple yrs. ago I made one like this for a great nephew and I made 130--6 1/2" blocks for a twin size quilt.
Will never make one with that small size blocks again especially making a quilt that big.
It was fun. Didn't really take that long to make all the blocks. I also used a light blue center strip.
I liked how all your center strips were 1" wide.....