Eh generally if you have something that tastes bad increasing the volume won't do you much good. DXM tastes too fucked up to be suitable for proper dilution I think.
How much should you take? We do not discuss here what you should and should not do but you can definitely figure it out for yourself easily. In
this DXM FAQ here on Erowid there is a special section for it,
click here to see your question answered.
You will have to read on a little, it is around the 8th or 9th paragraph when talk about dosage starts.
- first figure out how high you want to start, appropriate plateaus are the first and second. Check the table in the FAQ. For instance you may find that around 5 mg DXM / kg body weight suits you. Calculate the dose using your body weight, for instance 130 pounds or 65 kg means 5 x 65 = 325 mg DXM.
Then check the label of your bottle. If I am reading this correctly DayQuil Cough contains both DXM and phenylephrine: it says so here
http://en.wikipedia.org/wiki/DayQuil
So really this is not suited to use recreationally. 10 mg /15 ml is about 0.66 mg / ml.
The bottle probably contains 295 ml meaning you got about 200 mg of DXM in a bottle. So In this case getting 325 mg means drinking a little over one and a half bottle. Only one bottle should give you an idea of the effects though.
If I am correct and DayQuil contains phenylephrine, that will give you adrenergic effects such as increased blood pressure and vasoconstriction. Really nasty to have especially together with DXM. Make sure you have a DXM only product!!