Serving sizes?

Is there no way to add a different serving size to a custom food? I don't have the green + next to serving size. Was trying to make a custom recipe and was adding an ingredient but it's a bit annoying when the serving size only has 150ml but the product comes in a 900ml box and recipe would be to use one box. So do I have to do the math myself for cases like this (this one being easy obviously) or am I missing something here?

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