Sure isn't 100%.
It's trying to turn back into a solid at room temp even with very few crystals and going thru a heat bath.
How much pg is the bare minimum that works?
I'm not much of a diy'er, but I am a menthol freak. What I would do is ~~super~~saturate the pg. Take like 50ml or so and just keep adding ground crystals until it won't dissolve anymore. Then maybe top it off with another 10ml. Now you have a ~~super~~saturated pg menthol solution that you can add to your vg to suit your taste. Might only take a few drops per 10ml to get where you need to be or it might be (like me) that you'll need 25% or more of volume.
Just want to point out that that is not supersaturated. A supersaturated solution is one that contains more of the solute by percentage than can be achieved at standard temperature and pressure (25°C and 1 atm). Your solution is simply saturated, maybe even a little suprasaturated because you add extra solvent to it.
Most solutes can't form a supersaturated solution, but those that can are usually achieved by heating the solvent to allow more to solute to be added, then cooling back to room temp. Syrup is a common example of a supersaturated (sugar) solution.
Not trying to be a grammar Nazi, but this is chemistry, and terminology is important. Doesn't change the message of your post at all, just semantics.
Menthol is not soluable in VG at all, that being said making mixes aove 70% VG have provided me with an inconsistent vape and problems with menthol recrystilization.
Here is a thread I wrote about menthol concentrations.
https://www.reddit.com/r/DIY_eJuice/comments/2lr9gx/with_help_from_variablesin_some_menthol_info/
menthol is the only liquid I make in 50/50. Everything else is at least 70% VG, but unfortunately that just doesn't cut it with menthol.