I think the OP is asking what happens to the excess SPs from the flight that actually gets you over the line. For example, he needs 100SP, his next flight is 250SP. Do NZ ignore the 150SP or credit that into the requal year?

In my experience, I think it is, unfortunately, the former - that is, the requal year starts the day after, so the excess SPs are lost, but YMMV.

Hopefully, someone has some recent experience, or simply take your FF no. out of the booking and credit future flights (with smaller total excess value) to ensure you don't lose too many.