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DMT density varies a lot. Some is really fluffy, some is more powdery...so it's hard to tell unless you weigh some from the batch you have.
Some people will say it's reckless to give this advice...so I won't give it as advice...but, I was told to just take three big hits. The main issue is having too little IMO. I bet you'll chicken out the first time and take too little because you'll feel pretty funky after one hit if it works right. (Not you in particular, but I know I did...I first tried without a scale and always took too little.)
The main reason to use a scale is so you don't freak out because you assume you took way too much when it hits you. For me it's mostly a piece of mind thing. If you weigh it, you can just keep thinking...my job it so finish this bowl...smoke until there is not more smoke.
Some people have said about the amount to cover a dime, about the thickness of a dime...but like I said density varies. That could probably come out to anything between 25 to 100mg.
(If you use the same pipe each time, there's probably going to be some left in (unless you keep trying to vaporize it until there's absolutely nothing)...so even if you do weigh it out, after using the pipe a few times, there's probably more than you think in there.)