You have a lot of variables in play here.
First, the warmer the weather, the less critical the time off nest will be. Next, the nearer to hatch time, the chances are better for survival. the embryos start to make their own heat from their metabolism. With the large mass of eggs, they aren't going to cool off as fast, either..
I would bet this isn't the first time she has been on the wrong nest. You just haven't caught her before.
When I'm going to let a broody hatch eggs, I move her to an area that has the choice of only 1 nest. If I don't, she always ends up in the wrong nest.
Hope it works out for you.