I make my own menthol concentrate.
Yes you do add the 5g tub of crystals ( grind them first to powder) into a 10ml bottle.
It will fill it, but you add 10 ml of PG and it will dissolve. Even put bottle into a bowl of hot water to help it dissolve.
This is a concentrate, so just add 5% or 15 drops to 9.5ml to your ready made base.
I have read that also adding some peppermint or spearmint concentrate really brings it to life
Where do you get your crystals from?