Hey wineart, it sounds like you're close to 'D' day and that's great! My FOAF is no expert but she told me you need not be afraid. Even though it's natural and prudent to be careful with a new experience, my FOAF says you might laugh a bit at your yourself and your fear once you see what 'all the fuss is about.' It's good to be careful, but don't forget to enjoy.
Just to repeat, my FOAF is not an expert, but she had a few things to say about starting exploration. My FOAF didn't have a mg scale for her 1st launch. The FOAF didn't know how to vaporize efficiently and it took practice to learn. The 1st liftoff was on parseley in a small wooden pipe and it only produced some visual warpage, minor colors, and the appearance of spider webs on the trees she was looking at. She also felt spider webs on her face and a warmth inside. It was not even close to breakthrough but it produced a big smile in her heart since she finally was able to validate the magic.
My FOAF didn't like smoking on parseley since it made it harder to pull in the 3 hits. The FOAF made a glass pipe from a pyrex tube and learned how to get 3 good hits using a butane torch. It took practice and several non-liftoffs before getting the hang of it. My FOAF wants to purchase a GVG sometime soon but the pyrex pipe does do the trick until that happens.
As far as weighing accurate doses, my FOAF thinks it's nice but miligram precision is not essential. If you have a 0.10 gram scale you can easily estimate 25mg well enough to begin. In all likelihood, you will goof up your 1st few attempts because you won't be able to get 3 good hits. If there's a little extra in the pipe, you might not be able to vaporize it anyway due to technique. It takes some practice. While you're learning, you can practice with small doses and figure out how much spice it takes to launch. My FOAF thinks a secluded spot is best to begin. She never had a sitter but that wouldn't be a bad thing to have as long as they knew to stay quiet and give you space.
It's pretty hard to hurt your body with too much smoked DMT. Read the risks for yourself, but high DMT doses are almost self-limiting. Above a breakthrough dose, if you inhale too much spice, you will tend forget what happened to you in hyperspace. A single high dose of DMT won't necessarily traumatize you mentally. It's not like (say) LSD where you can cause a big (and long) problem for yourself if you overdo it. Smoked DMT lasts about 10 minutes almost regardless of the dose. If you dose too small you don't break through. If you dose too high, you won't remember. This aspect was also reported in Strassman's Spirit Molecule book. When he gave very high doses, people emerged from hyperspace without remembering what happened. My FOAF says it's a strange but not unplesant experience. Anyway, your technique is going to be more significant than the difference between 25 and 30 mg loaded.
These words could sound too casual. No one should be casual about DMT, and for those that are, it will surely teach some lessons of respect. You will want a comfortable, safe set and setting. BUT, it's OK to relax, and practice, practice, practice....