I would dilute it 7 to 1 or 8 to 1 with distilled or purified water and take 1 of the 30 ml dose measuring cups for a 3.5 to 4.0 mg dose. The diluted DXM should be stable in the fridge for at least a week.

In my opinion the other ingredients you described are inconsequential at the levels remaining in the diluted solution.
Robert