As of the end of 2015, Tesla had sold 50K cars in the US. They're predicted to make another 35K US sales in 2016 and 50K US sales in 2017. So the worldwide sales have no impact on whether you get the full US incentive or not. The US Roadster, S, X, and 3 sales do impact whether you get the full US incentive.
That means that at the start of 2018, they'll have made 135K US sales.
The incentives don't immediately end when Tesla crosses the 200K mark. The full incentive continues until the end of the quarter after the quarter when the 200K mark is crossed.
If they manage to cross the 200K mark in Q1 2018, that means they're making the cars very quickly - they made at least 65K cars in Q1 if they managed to cross that mark then. Presumably, if they make a different amount of cars in Q2, it'll be because they're going faster, not slower.
So in the scenario where they cross the 200K mark in Q1 2018, at least the first 130K Model 3's will have the full incentive. That's the number of reservations that had been made as of the Model 3 event ending last night (it has climbed to 150K since then... those people may not get the full incentive, but they'll be part of the 130K+ who get the 50% incentive that lasts until the end of 2018).
If Tesla crosses the 200K mark in Q2 2018, it means they made at least 65K cars in those two quarters. I think it's safe to guess that they'll make another 35K in Q3 2018, which covers at least the first 100K Model 3's with the full incentive. That covers all but 15K of the people who made their reservations before Musk made his announcement partway through the Model 3 event last night.
So the faster you think the 200K mark is going to be crossed, the more people will get the full incentive.
On the flip side - the slower Tesla is to make your car, the more time you have to save up so that you don't need as much of an incentive.
Also... I kind of suspect that the projected S and X sales for the rest of this year and next year are going to be off. I think the fact that we've already seen the Model 3 is going to put downward pressure on the sales of Tesla's existing cars. So they'll start the Model 3 production with even more space until they cross the 200K mark. I think last night was "Part 1 of 2" to kind of mitigate this and keep sales of the S and X from plummeting... but you know how they talked exclusively about the base Model 3? The base Model 3 sounds pretty much identical to the base Model S, except it costs half as much. I suspect that the high end Model 3 is going to sound like the high end Model S, but at half the price. They're saving details of the high end Model 3 to have an illusion that the Model 3 is going to be inferior to the Model S, when in fact that Model S is going to be discontinued entirely and the Model 3 is going to be its vastly cheaper successor.