there are two spectrum, the fun spectrum and the harm spectrum, which are related very closely, and every person has to place a line somewhere across the two. picture it like this:
the fun spectrum
studying for tests--------road trips-----------getting high-----------skiing---mcdonalds playplace
the harm spectrum
jumping off buildings--------unprotected sex-----------playing sports--------watching tv
there is no equation for what is too far and what isnt far enough since everyone is different, but its important that each person think critically about why they do the things they do, and is it worth it? for your example id say smoking a joint and smoking a cigarette are fairly far apart and not easily compared, since smoking a joint is a lot more fun, and less dangerous, than a cigarette. that doesnt mean either one should or should not be done, but, in terms of the above diagram, an individual is going to be more likely to smoke a joint because its slightly farther to the right.
as long as a person thinks about the ends and means of an action and makes their own informed decision, i dont think they should be criticized for the choices they make.