It's quite effective, as long as there's *some* nicotine; all by itself, it doesn't do much at all, but with some nicotine, and a fair amount of PG, it does a GREAT job.
It's hard to say exactly how much I use, though. I learned very early on that it's very easy to get too much, and fuck up a whole mix pretty badly, so I started making a pre-mix "sweet flash", using some Flash, nicotine, and a bit of sweetener, mixed with PG. My first pre-mix, when I was at 10mg, was a 10mg nic solution, using 20% flash and 2% sweetener. Later, as my nicotine level came down, I made a new pre-mix, 8mg nicotine, 28% Flash, and still 2% sweetener. Currently, I'm using a premix which is 3mg nic, 35% Flash, and 1% sweetener. With the pre-mixes, I use anywhere from 6% to 9% in a recipe -- currently with my latest pre-mix, I'm using 7%.
The pre-mixes were helpful because at first I was using WTA, which is 100% VG, so I needed extra PG to bring me to my preferred 80%-85% PG, and since I was using 10% WTA at first, I also wanted to make sure that it stayed sweet enough, hence the 2% sweetener in the pre-mix. The WTA has been gone for a while, and this summer my tastebuds finally started waking up, so I need a lot less sweetener than I did at first -- but since I'm currently at 3.3mg nic, I need a bit more Flash to still get good TH.
My recipes for the various Flash pre-mixes are at
https://ecigvape.com/recipes/crypticvapes -- Sweet 3X Flash, Sweet 4X Flash, and Sweet 5X Flash -- along with all my other recipes.
Andria