Hmm... it's hard for me to say exactly, because I make a pre-mix of PG, Flash, sweetener, and nicotine, My pre-mix currently 3mg, 35% flash, and 1% sweetener... in PG. But I started with 10mg, 20% Flash, 2% sweetener. I've had to adjust my pre-mix as I've lowered my nic level, and lowered the amount of sweetener I use; with my current pre-mix, I mix it in the recipe at 6.75%.... I'm not very good at math, but that appears to be about 2.3% Flash; the earlier would, I think, have been about 2% Flash -- since I was using more nicotine, I didn't need quite as much Flash.
But if you're just going to mix Flash directly into your recipe, without pre-mixing it, I'd suggest using no more than .5% to start, and see if that's enough; you can always add more if you need to, but it's easy to mess up a whole recipe by using too much; it really does cause coughing, due to the tickle in the throat, if you use too much.
Andria