Basix : Dissolve 10 gram ( sorry, me metric ) in 10 ml PG. Will need a hot water bath, some patience and lot stirring.
Of that mixture 3% into your base liquid is ample for a distinct menthol kick. 2% if you like easy, 0.5 to 1.5% to add to flavoured liquids for a bit of tweaking...
4% strong...