How do you know you took enough DXM?

I took about 300mg and It's my ffirst time I'm not sure if this is how it feels.I feel slower in a way and I see things just a little closer nd vividly but I'm not sure If i'm just tired or if it's because of the cough syrup. Idk maybe asking this question is proof i took enough. Please dont reply if ur just gonbna say how stupid and desperate i am for using DXM