> I see in various websites and publications that people used different formula, e.g., 2*t_stat / sqrt(n-2)
There are different variations for Cohen's d. You can pick up whatever you prefer.
> my t-test is a paired one, should I account for that?
To me a paired t-test is basically the same as a one-sample t-test: you can simply obtain the difference between the two conditions for each subject, and then run a one-sample t-test.
Gang