I am more confused by the plus than by the ratio symbol, since I have never seen it used that way. Numbers separated by ":" clearly indicate that the numbers are meant only to indicate proportions (except in the aforementioned professions). Numbers separated by a "+" indicate that the numbers on either side are the actual amounts to be used in measurement. Anyone who has passed 1st grade math knows that. Using an addition sign to indicate a ratio, against common conventions, is not clear.
And neither of them mean anything without stating what the numbers on each side of the symbols mean. You don't just say "1:3," because it doesn't mean anything by itself. You'd say something along the lines of, "Stock to water 1:3," "One part water to three parts stock," both of which make it abundantly clear. If you say "to" or "ratio," it is immediately clear what is meant.
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Older formulas would often say "dilute 1 part of developer with 3 parts of water." A bit wordy but totally unambiguous. The problem is that some mathematically challanged people do not see the ':' as indicating a ratio. People on APUG are fairly frequent in asking dilution problems. Let's make it easy for them. For this I personally prefer the '+' notation.
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