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Why not simply add 213 to the previous number? If the result is more than 999 simply wrap around to 000. For instance, say I enter in 850 for the previous midday number and don't enter anything in for the evening number - one of the "projected" numbers would be 1063. Because that's bigger than 999, we simply "wrap around" (also called 'mod' or 'modulo' in math/computer speak), and get 063.
Would that work?
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