Your quilt is very pretty! Don't give up on this technique...try it again and again using scraps and see what works best for you.
After looking at the tutorial, it appears that the binding is much wider than yours, but then the tutorial shows white on black, so maybe it just seems wider.
I agree with ckcowl in that the zigzag fabric seems to dominate and detracts from the cathedral effect a little bit.
I've been working on the 10 minute blocks using the cathedral window method on them. I found myself using a slim ruler to measure how far I fold back into the center, then simply iron the center and let the iron push the fabric into the corners. It seems to work very well, and the sides of those blocks are not on the bias. I find that I can push the center in quite far if I wanted to make the curves deeper.
Your helper has beautiful eyes!