This is the toughest time in the 34th America's Cup. It's the time of educated guesswork, when all that technology and science combine with good old-fashioned intuition.
There were signs on Monday that the Kiwis might have got it a little wrong. Not only did skipper Dean Barker surrender the advantage in both starts to Oracle Team USA skipper Jimmy Spithill, their boat and crew work in Race 4 seemed a little less convincing than previously.
The best guess is that the New Zealanders misread things.
The winds were forecast to be light but, by Race 4, were gusting over the 23-knot wind limit. San Francisco is not an easy climate to read even when you know that the wind will build during the day.
Even worse, decisions have to be made by 8pm the day before - so their weather forecasting has to be spot on.
What may have happened is that they moded their boat for lighter airs, making it faster but maybe less stable.
Then, when the wind blew, Oracle took advantage. That theory is borne out by the fact that the wind blew hardest during the third leg of Race 4, the upwind leg.
That was when Aotearoa, dominant upwind in the previous three races, faltered. Oracle actually won that leg, extending their lead from 5s to 16s.
At one stage, the Kiwis attempted to foil upwind to add a burst of speed; it turned out to be a symbol of their inability to catch the American boat with the bows rearing up as either the foils weren't set quite right and/or the wind caught the bows.
Today's weather forecast is for stronger winds - 18-20 knots on an incoming tide. So Team New Zealand again have a ticklish decision to make in configuring the boat for a range of winds from 10 knots to 23.
Best guess for today and for the Cup match as a whole is that Emirates Team NZ do have a faster boat - but perhaps not by much and perhaps they have re-jigged the boat so much that they are not as adept in stronger winds as once they were.
Oracle so far look better in stronger breezes. Theirs was supposed to be a boat moded for the supposedly lighter airs in San Francisco Bay in September.
The reality is that it is still too early to tell. There's not been quite enough disclosed form yet.
The AC72s are still developing and, as the sailors have been at pains to say every day, they are still learning about how to sail them. Barker's comment the other day - that the boats will be faster in a week from now than they are now - is relevant.
So there are too many variables for armchair critics to be definitive either. But, if gun was held to head, the best guess is that the Kiwis have an edge in upwind speed; Oracle have an edge downwind, maybe particularly when it blows.
Upwind speed will still be important and is still likely to be the single most telling edge, as Race 3 suggested when the Kiwis came through from behind to win.
As for the variables, the start and the rounding of the first mark are vital elements. As Races 3 and 4 showed, the way the boats are set up will be important as will the weather - and, most of all, it is clear that crew performance is a big factor and can win races.
And that is the saving grace of the 34th America's Cup... all that science and technology and it may yet come down to the sweaty guys on the grinder handles and their power and timing as they go through 10 or 11 demanding tacks.