Agree with cadet
250-300 w'd not really test Sri Lankan batsmen. They bat deep. They are unlikely to collapse again a second time and with 2 full days remaining (assuming we bat atleast all day tomorrow..) and a first test series victory at home against Pakistan in sight they can afford to cut out all unnecessary strokes and go for the target at a leisurely pace.
So Pakistan really need to score 450-500. It is still a day 3 pitch. There are no real demons in the track and if our batters bat sensibly & with discipline there is no reason why we should not be able to survive tomorrow with 9 wickets in hand