I don't see the reason for making that big a score... keep in mind that it's a delicate balance that needs to be maintained to show the opposition that they have a challenging total that is doable in the time that they have remaining. So if Pak finishes its batting or declares around lunch on Day 4, SL will have a day and a half - which is just enough to make a score of 350 to 400, and they'll be interested in trying to win this match. Our bowlers will then have a good contest on their hands.
Anything more than 400 will put SL on the defensive and they might just try to draw the match. Remember that their batting hasn't exactly clicked against us either, so they're going to be cautious as well.
I meant 450 score and 300 lead atleast. Remember the 150 first innings deficit