Hello sue thanks for your idea the way i done it was similar and put the measurement of PG into graduated ml cylinder then pour crystals straight into cylinder then hot bath to dissolve then poured straight into 50ml dropper bottle all ready to use. Will give it a try for cherry,BlackBerry,blackcurrant menthol blast. if strong it will be Lift off.ha.To be honest you don't even need a pestle and mortar.
Put the crystals straight in a bottle, add the PG put the cap on then get a small container and do a hot water bath.
After about 2 minutes vigorously shake the bottle then change the hot water then after another minute or so you will have your menthol concentrate ready to use.