Trying to clarify here. Was the vial made as a microdose vial originally? Or are you trying to use LSD in a microdose style?
If it was a microdose vial to begin with, I wouldn't worry tooooooo much about it being concentrated to 3x potency. Most* microdose vials are 5-15ug, so the dose would be 15-45ug, which is generally considered a pretty light dose..
However if it was a standard vial (which are generally dosed at 100ug a drop) you'll definitely want to be careful. Because like
@Esperighanto mentioned, if it's evaporated to 1/3 of original volume, its going to be 3x more potent.
300ug is a face melting experience for most people.
Thank you for being responsible. Here are a couple of options I did not mention. You could easily reconstitute it back to the original volume by adding vodka or some other spirit. That would at least take some of the guess work out. (In the US, a vial is usually* 2.5mL or 100 drops approximately...
If it's a regular* vial and you're trying to microdose with it, I would reconstitute it back to original volume, and then add 1 drop to 9 drops of vodka/spirit for 10ug drops.
Alot of assumptions here unfortunately.
However if you can spare a few bucks there is a company that sells quantitative home LSD test kits. I am not in anyway associated with them, but I met them a few years back at a conference and they seem to have a good scientific background as well as reputation.
www.miraculix-lab.de
I've not used them, but they claim to be able to gauge the strength of LSD and many other substances.