OK, I'll try to answer all the questions here...
It's unnecessary to leave 50+% of your space free (I generally suggest more like 20%), and worrying about external-vs-internal cylinders is pretty much unnecessary. And while it's best to use a destination volume the same size as the source, that's mostly because data grows, and using a drive that's too small means you'll run out of space.
Just because I recommend not backing up the Sandbox doesn't mean you don't back up your original drive. You'll always do that, and you should do it pretty frequently (and it's always better to move more copies than fewer). It's also pretty important to install the updates to the main volume after you've made sure they're OK.
Remember: if you find out three months after you've installed an update there's a problem, you can always archive-and-install back to a much earlier OS release, or reinstall an application. The point of the Sandbox is to save you time, not isolate you forever - "at your leisure" doesn't mean "years later" - that'd be silly. But if you don't, you can still simply update/install/whatever the original volume without the Sandbox. They're updates. You can get them again.