Phen, the ideal would be to weigh the unopened package (with your block, that's an excellent addition) deliver the entire contents into a small amount of solvent, then weigh the empty packaging. The difference is the mass you added to your solvent, probably just shy of a gram. Put the empty packaging into a sealed container.
It's easier to dissolve things into a small amount of solvent first, not the final volume you intend. If you've ever tried to thicken a sauce with cornstarch, you know you dissolve it in a little bit of cold water first, or else you'll get lumps you can never get rid of. Dissolve the 0.995g in 5.0mL of 50% PG if you want (assuming it's readily soluble). Once the powder is in solution it's much much safer to handle.
Mix this by sealing it and agitating, don't use a utensil. After like 24 hours of agitating (off and on), maybe some gentle heating, go ahead and start diluting it further. You would QS to 10mL first, maybe (adding a gram of solid to 5mL of diluent will make the volume go up). Your solution is now 1000mg/10mL, or 100mg/mL. That would be as many as 400 doses per mL.
From there you have lots of options, including just spiking the entire contents into 90mL of diluent. Store that as a stock, 10mg/mL. One point from that stock (100uL), in a syringe, would then be a strong dose.
Or you could divide that 100mL into four separate stocks, at a 1/10 dilution, so they're all 1mg/mL. It really depends on your intended instrument for final dosing.
But please please be careful. This stuff isn't as lethal as fentanyl, but it will still easily kill you or your pets or the garbage man. And weighing the package, opening and pouring into diluent, should be as fast and careful as possible. By starting with the entire contents (the largest mass) you will have much fewer problems with error, and a measuring error of 1-2mg becomes not nearly as big a problem.
WHatever you do, don't snort this stuff.
Edit: and for everyone ordering this shady stuff, don't purchase "powder" of a substance that has active doses in the fraction of a milligram. It will easily kill you, and very likely you are getting conned by these vendors.