IMO making 450 million is not the ceiling for it. Sure it's possible it won't make 450 million, but I would say not likely.
Dory was the 18th fastest to 100 million
17th fastest to 150 million
13th fastest to 200 million
9th fastest to 250 million
7th fastest to 300 million
6th fastest to 350 million
It still has just under 20 million to go to 400 million, but even conservative estimates would have it reaching that before Friday (when Secret Life of Pets comes out) putting it as the 5th fastest to 400 million. It should see it's largest drop of any weekend coming up this weekend, but after this weekend we will have a better view of what it's final potential is. If it makes 20 million or more this weekend, then I think 500 million is a possibility. Dory is on Day 18 right now, Shrek 2 made 139 million after day 18 -- if Dory were to make that it would be over 500 million. Toy Story 3 made 114 million after Day 18 -- which would put it just short of 500 million. So far Dory has made more than toy Story 3 on 17 out of the 18 days (yesterday being the first time Toy Story 3 made more...but on day 17 Dory made over 50% more that Toy Story 3). Despicable Me 2 made 100 million after Day 18, Finding Nemo made 144 million, Inside out made 106 million, and Zootopia made 136 million. Based on these numbers I would say the 500 million is achievable, with the ceiling being around 510 million. (note - only 6 films have made 500 million, so that is a lofty number)
I will be really interested to see what it does now with some big competition.