https://steamcommunity.com/stats/SpaceChem/achievements according to steam achievement "beat the game", it's actually 4%. I do agree that it makes it a difficult game to finish. Then again, my point is that I think it's fine not to be able to finish a game, especially a puzzle game which have no story. You just have to spend more time on the puzzle you are stuck on, so in a sense, you get more game time for your money :)