Most of what I mix up for my wife contains menthol, which I also DIY. I pack a standard 1oz bottle with menthol crystals, and then fill with PGA. The resulting liquid is what I use *sparingly* to add menthol to a mix.
For example, in a 120ml mix-up, I use 0.5ml of the menthol liquid. Not much at all, and yet enough to know it's there. For just a plain menthol, I might use 2%.
And yes, I did get stupid one day and took a hit of straight menthol, back when I used PG instead of PGA...
Just about took my head off. I've done it a few times since., when I've got a raging head cold. ONe hit is guarenteed to instantly clear the sinuses.