if you live in the us, then exactly what is in the package is what's on the label. if it's something that you have to add milk to [a boxed pasta dish for example], the label might have a second column that says "As prepared" with whatever percent milk, but it has to have the plain, just what is in the bag/box info. so use that plain info. then you just account for the soy milk separately. if math isn't your thing, then just enter it in as a recipe with two ingredients. one would be the packaged item. the second would be the soymilk. then all you have to do is measure what you had.
What packaged foods are you referring to? The ones that come to mind for me - cereal, for example - include nutritional content for just the cereal; milk is added over and above.
Do you mean instant mashed potatoes, or something similar? You could bring up the nutritional content of the milk recommended in the directions and compare it to your soy milk label. Reduce total nutrients by the difference. That's the easiest/only way I can think of. The differences will generally be sort of minimal, though. I'd just track it as-is, and know I've got a tiny cushion not reflected in the #s, but I'm lazy like that.
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