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Its cheaper and easy enough to just use crystals theyl dissolve better in pg, I put about 2g in when making up 250ml of juice. I put nic base pg in bottle 1st, then pg, then flavours then grind up the menthol to a fine powder in pestle and mortar. Tip it in the bottle and shake. At this point theres no vg in there. It dissolves better without vg. When its dissolved add vg and shake again.
Straight menthol named flavors will be exactly that. You could buy crystals and dilute your own, but why waste the time doing that?
Some people like a much higher concentration? Although given there's a limit to what will remain in solution in a 96% VG base, probably not totally useful to most. (I suppose if one dissolved the crystals in ethyl alcohol, that might allow getting more to remain in solution with minimal PG. But you need to be excessively sensitive to PG, which is like less than one in ten thousand level sensitivity.