Question

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I have done all 3 before 180 over 2, 180 dash 2, 180 slash 2. I think any of these is acceptable as it denotes a combination strength.
 
I don't like this because it implies a ratio. It could be used in a case like Amoxicillin 400mg/5mL because that's a ratio that could be reduced mathematically to 80mg/1mL and still be the same product.

I agree with this. It is not 180 divided by 2, which is what 180 over 2 would mean. It's a small thing, but still.