Seed Dish And How Much Seed To Feed

When I see the large round dishes of seed on the bottom of cages and avaries I always wonder how do you keep it clean. The finches sit in it while they eat and also poop in it, does this hurt the finches if it sits in there all day? I use the seed cups that hang on the side and they usually manage to get a little poop in it and I scoop it out if I see it while I am home, but I would like to use the large round dishes on the bottom, it would save time from having to change and fill each seed cup, but didn't know if the poop was a big problem in these larger open dishes.
Also, do you go by the "rule" of 1 0r 2 tesp. of seed daily? I don't, I go by how much has been eaten and give that amount when I refill the seed cup. My societies are the biggest eaters, from seed to eggs they eat everything, so 1 or 2 tesps would not last all day. That is why I am always running out of seed! Would they survive on less seed?