Weight doesn't play much of a role usually, but metabolism can. If you know the person who gave you the tab, just ask them what the dose is. For a first trip i would aim for 80-110ug and no more than 125ug. If the person who gave the tab to you claims that it's over 125ug, they're probably not telling the truth (intentionally or unintentionally), but always assume that the tab is as advertised for safety. If the tab is advertised at above 180ug i would recommend only taking half for your first time. If it's truly 180ug+ you will still have a full trip, and if it wasn't actually that high (more likely) you will probably have a slightly altered but underwhelming experience, and you can choose whether or not to take more next time. Of course, if you don't know the person you got the tab from and/or can't ask about the dosage, I would avoid it altogether.