My resting metabolic rate, the calories needed just to run my body at rest, is roughly 975 to 1000 calories. I currently eat between 1450 and 1500 with exercise, on a weekly average. I will have higher days when I have significant exercise or I just blow it. Last week when I rode 57 miles I had about 1900 calories that day. But my weekly average still stayed close to 1500. Last week it averaged 1534.
When I was heavy my resting metabolic rate was roughly 225 calories more than it is now. That is because I had more body to support. :) I think that I always want that additional 225. :)
As far as variability, the "hacker's diet" has a good discussion of the fluctuations people have that basically are meaningless: http://www.fourmilab.ch/hackdiet/e4/signalnoise.html

