if it needs to get to 600 it should have tracked at 10+ million on Monday. Long term tracking is now at 560-580 Million, that is just 20-40 million above TDK
I don't see this at all. It has held better than the Dark Knight almost every day so far, and is currently running about $58M ahead of the the Dark Knight, yet you think its legs will just drop off resulting in it loosing ground for the rest of its run? The Dark Knight had fairly similar third week competition as the Avengers will have. The Mummy 3 opened to $40M which will probably only be a little lower than Battleship's opening, and Step Brothers' second weekend take was similar to what Dark Shadow will make this weekend. Your competition reasoning also breaks down when you consider week 4 is memorial day weekend. What it loses in later weeks will be made up for on Memorial day. These movies are 90% done by week 5 or 6 anyhow. As long as the Avengers stays competitive through Memorial day, poor late legs are going to make the difference of maybe 5% of the total. Also, Summer can support a number of movies. No reason why the legs for the Avengers will be that poor.
If the rest of the weekdays follow today, the Avengers will make $27.1M through Thursday, putting its total at almost exactly $400M. A 45% drop on its third weekend (which would be steeper than the Dark Knight's 43% third weekend drop), would give the movie $56.7M, putting the total at ~$457M. Another round of 50% weekday drops (which would be high. Iron Man 1 was dropping less than 40% that week) and a 40% weekend drop on the 3 day memorial day weekend (again high), would put the movie at $504M. And that was being conservative.
If you change this weekend's drop to 40%, the following weekday drops to 40%, and the Memorial day drop to 35% you get $518M before Memorial Day Monday. And none of those drops would be particularly amazing for a well liked blockbuster. Iron Man 1 had slightly better drops than this scenario, and it had a $55M opener in its third weekend (Narnia 2) and a $100M opener in its fourth Memorial day weekend (Indy 4). At worst, Avengers will have similar competition.
EDIT: Regarding competition killing legs, Summer 2008 was probably as packed as this summer will be. Most weekends had a $50-60M opener through June and early July leading up to The Dark Knight. Despite this, Iron Man made 21% of its gross after the Sunday of Memorial day weekend. If the Avengers hits $510M by the Sunday of Memorial Day weekend, and has similar legs it will reach $640. To reach $600M after hitting $510 on Memorial Day weekend, Avengers will only need 15% of its total gross going forward.
And just to drive this competition notion into the ground even further, Iron Man made a larger portion of its earnings after its fourth weekend than The Dark Knight did. Despite the fact that it faced major openings almost every weekend, and the Dark Knight coasted with little competition.