Anyone who thinks the declaration was early is dreaming. As far as I can make out, only one team has ever chased 350 or more at a run rate greater than 3.6. If SL can pull off one of the greatest chases ever then it won't be the fault of the declaration.
Statistically I would guess that the most likely result is NZ win but my gut says SL will bat this out. If the game is still in the balance come tea time the new ball shortly thereafter will be the critical factor.
The problem with this viewpoint is
A: We just saw a pretty long standing world record broken in regards to run scored by a single partnership (Against SL). Williamson and Watling were able to bat 110 overs on this wicket and put on 365 runs, the wicket has not deteriorated any further since Day 3.
B: We saw last year on the same ground a similar partnership in the closing stages of a test match with Watling and McCullum indicating that batting here in the final days is pretty easy, irregardless of the bowling attack.
C: Yesterday afternoon, our main weapons looked practically toothless save for a couple of balls that beat the bat, before the wicket fell to a mistake against the spin bowler, Sri Lanka were keeping pace with the scoring rate.
Had we batted another 10 overs, put on another 35-45 runs and made it 100 overs to score 420+ then you've effectively removed any chance Sri Lanka have of winning the match, can afford to attack all day long without any risk of losing the match and set ultra aggressive fields.
It'll come down to where Sri Lanka are at with about 30 overs to go. If they're chasing say 150 with 30 to go, six wickets in the bank then they'll have a run at it. And they'll probably be favorites by that point too.