Great looking quilt! I'll be it can take you down to zero... I have a similar DIY quilt that I was more than toasty in at the recent Northern Hang. It also has 15oz of down and weighs in at ~24 oz. Pretty similar dimensions too. It was my first real sewing project, so I also made a bunch of mistakes. But, as you said, thats how you learn!
Looking forward to your testing trip report!