At Adepticon, we tried to get all the boards reasonably close to 25% covered, some were a little less, and some were a little more. I'm not writing today to discuss whether 25% coverage is the way to go, as far as I'm concerned, that is the way the main rule books says to do it. Also, one would think that that was the way it was play-tested as well. Ultimately, it is up to the players to decide where on the spectrum of vaccuum of space to space junk yard they want to battle.
What I would like to ponder though, is the options to get to 25%. We tend to use medium and small chunks of terrain which litters the board, about 25% of it actually. What we discovered during Adepticon, where all the great metas converged was that other people get to their 25% by using less pieces because they are much larger. Some people said our boards were "mazelike" which was very interesting.
Part of the research and development for next year will to have a variety of board styles, some with many pieces like we had in 2015 and others with fewer but larger ones.
How does your gaming group grapple with terrain coverage?