Some numbers

Sedentary Lifestyle
RMR 1,939 kcals
BMR 2,012 kcals

Using the higher of the two, cos I am not daft
Down Day 424 kcals (20%)
Up Day 2,000 kcals

Being sensible and hungry, I shall round that up to 500 kcals per Down Day.

So I need to find a lot of low kcal recipes.

Porridge
1/2 cup made with 50:50 milk and water plus cinnamon = 200 kcals

1 pan egg and veg = 200 kcals

So I just need a 100 kcal lunchy thing or add 100 kcals of exercise on Down Days!

Lunches could be cous cous or quinoia and veggies!
Check bean books