Should I stir my sourdough starter before measuring?
Rather than worrying about whether you 'stir down' the starter before measuring by volume, you should really be measuring by weight. Kitchen scales are cheap and incredibly helpful, especially for any kind of baking.
Regarding feeding: there are almost as many different feeding ratios as there are sourdough starters. None of these are set in stone. A common  ratio is 1:1:1 starter:flour:water, again by weight. Thus, if you keep more starter, you feed it more.