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I typically use ws23 at (0.5% or .15g) for 30mls for noticeable cooling, but I don't use it all the time. If your used to using it, you might want to use 1%. As for drops it's hard to say since some bottles drops weigh 0.02g and others dispense 0.05g drops. So it could be anywhere from 3 drops to 7 drops for 0.5% and 6 drops and 14 drops for 1%. So I would start at the low end and keep adding until your satisfied with the result. You can always add more, but can't take it out.
It's a very personal preference type of thing. Heavy menthol vapers tend to build a tolerance to the stuff and use higher percentages. Start at 0.5% and work your way up. I've been vaping menthol/ cooling juice for over 5 years and currently use WS-23 30% @ 2% in my mixes.
People tend to add 1% to 1.5% WS-23 to their e-juice. Strangely the percentage commonly used is the same regardless of how strong the WS-23 is. People get WS-23 crystals and add them to PG to make a WS-23 additive in 10% to 50% strength... but still use 1% to 1.5% in their e-juice regardless of the strength of the WS-23.
So in a 30ml bottle I'd try 0.3ml, or 0.09g if you have scales, or about 3 to 5 drops.
I could never use 1%, let alone 1.5%. It's almost too much for me at 0.5% even. I'd start lower than 1% at first. Can always add, can never take out.
I use 1.5%, but my WS-23 is diluted to 30%. So the final % is really more like .5 . I think the different dilution %s are where the confusion is.
Mine is 30% (most bottles of WS-23 are, iirc). 0.5% of that is cold af for me. You do build up tolerance for it, so if you use it often, you will need more and more over time, I think. I'm not much of a coolant guy, so I only use it in recipes that I really like with it.