After the flip, the freshly milled corner is placed at zero and used for WCS zero for next setup. Perhaps I’m missing something, but it seems to me that exact stock dimensions do not matter when using this approach, because I’m milling two sides and facing off the top in the first operation, and then placing the known corner at zero.
The whole purpose of doing this approach is to be able to work with imperfect stock — in principle, your stock doesn’t even need to be square, as long as you have enough of it to cut out the reference faces and your model.