Do yourself a favour and titrate your dosages very slowly and carefully.
I agree with testerpester that volumetric is the way to go.
Just do not dissolve all the substance you have at once, use a decent scale to measure a small amount and attempt a small test batch.
I would go for a 10mg/1 mL concentration at first, and use a 1ml syringe to measure out the needed quantities.
In case you haven't done that before, it doesn't mean you are putting 100mg in 10ml of water, you need to use a different method.
You will need
- a 0.001 capable jewellery scale,
- deionized water, but low mineral bottled water will work ok as well just do not use tap water
- a clear dropper bottle of adequate capacity, made of glass
- some kind of stirrer instrument, made of glass
- some tape
- a piece of clean smooth paper to hold your drug while you measure the weight, could be rolling paper in instance
- a 5ml syringe, no needles required
- a 1ml syringe, no needles required
So, tare the open bottle on the scale and add exactly 10 grams of water, and then mark the water level VERY accurately placing some tape on the outside of the bottle.
Now remove about half the water, weight 100mg of the substance on the paper and drop it in the bottle.
Stir until the drug is under the level of the water, no crumbs floating.
It is possible that the substance will be dissolved with some stirring, but if that does not happen quickly close the bottle, and warm it up by placing it in a pot with hot water and stirring occasionally
Do not use flames to warm the bottle. Do not microwave.
Once dissolved, just add water using the 5ml syringe until the liquid reaches EXACTLY the level marked with tape.
Stir well and close the bottle with its cap.
Now, the solution is ready and you can use the 1ml syringe to extract the required amounts.
The full 1ml syringe will hold 10mg of active drug, so 0.1 on the syringe will be 1mg.
I would start with less than 1mg, as you are not sure what drug it is and what is the purity.
Space every test by several hours and increase doses slowly and carefully.
Every time you intend to measure a dose from the bottle, hold it in a well illuminated spot and check for chunks or powder suspended in the liquid, if you spot any you will need to warm up the bottle like described before taking the dose out.
Even if you do not spot chunks or powder, stir well every time before extracting.
Be very careful at all times, please if in doubt ask questions, do not rush the procedure as it could cost you dearly.
Good luck.