I would use pure alcohol, no water. But I should add I'm just beginning to use volumetric dosing so I don't know the particulars yet.
Stability-wise, i'd guess you could hold it for a few weeks, maybe months, but not indefinetly.
I have a question myself though; you just assume you can dose at 10mg/drop ? I've been wanting to try this method with 5MeOaMT but I don't know how much alcohol is needed for a certain amount of material.