I used to plan everything in advance - down to the individual meals. These days - i plan aronud the type of week i think i'm going to have work wise (i sometimes end up getting home quite late), and ensuring that whatever happens I have something easy and quick in the house. I also have an idea of what i'm going to have each day, but am flexible enough to change that around if things change. Anything up to the point i get home really i can change as my lunches tend to be relatively neutral on the green/red scale of things.
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