I didn't set the ratio myself but just dragged one element to the other (tried both ways), while holding down ctrl, then picked "Aspect Ratio" in the dropdown list - no idea how to do it any other way. The UIImageView is a child of the UIView and they both have their "spaces" set to their own superview in the size inspector. The most outer element is a UITableViewCell and I wanted to make IB (it was complaining about a missing constraint) happy by setting the full size like that. Even if there was a zero division, it shouldn't crash but give some type of error message.
After the new OS version was done installing I ended up with around 35gb but it was still complaining about not enough free space. I deleted two old SDKs for versions I'd never been using (why is it even keeping/installing that stuff in the first place), which got me to around 43gb and it finally worked. If I'd stopped the download (took the whole day, downloads from Apple's servers are always so slow!), I probably wouldn't have been able to start it again because of the space problem, which happened to me before. I had to call Apple and they helped me clear some cached data. They really have to find a better way to do this, it's a problem every single time and this machine isn't even used for anything else apart from Xcode!
When I started it up just now (oh yay, they changed a couple of icons, so worth it!) a new error showed up for 3 of my UIViewControllers:
Failed to render and update auto layout status for ViewController.... (ID): Agent crashed
Great, I got the same error before the storyboard broke but atm I'm using the old backup one.