There is nothing official. Some people try to figure it out by logging win bonuses for various strength bases, and watching their own wins bonus go up to see how various upgrades affect it.
From what I've heard, people say defense counted for maybe 70-80% of the total matching, though that was before they tweaked it to give a little more weight to walls and heroes, which previously accounted for very little (as far as anyone could tell). It's really just educated guessing, and there are other factors that go into it too, like how long you've been searching. And that's the biggest thing I hear...if you're having trouble with mismatches, restart the matching every few minutes.






