Let's think about it for a minute. What if, instead of only doubling the batch size, you wanted to make ten times as much? Say your initial recipe calls for 10% flavor. If you multiply that by ten, then your big batch is now 100% flavor, and you have no room left for anything else. Success! No mixing required, just dump the flavor straight in your tank and vape away! Oh wait...
10% is 10%. It doesn't matter if it's 10% of 15ml, or of 15L. It's still one part in ten, or ten parts per hundred (that's literally what "percent" means: "per centa" or "for every hundred.")
The only way that doubling the flavor percentages makes any sense at all, is if you double all the other ones as well. And then your recipe would add up to 200%. In the end, I guess you'd get the same result, but it's not the "right" way to do it. Probably better just to leave the percentages alone (all of them) and just multiply each one by 30ml for the final amounts, instead of by 15ml.
I am of the mind that "getting the right answer" is less important than "knowing how to get the answer." I guess the math comes more easily to some folks than it does to others, but the only reason I would ever use a juice calculator would be to check my work. A regular calculator, sure, but I don't need a juice calculator. (Maybe I'll change my tune once I actually start mixing some extraordinarily complex recipe, but I doubt it. I mean, a percentage is a percentage. Just make sure they all add up to 100, and multiply each one by the final amount.) I would say the opposite, actually: "You can use a juice calculator to make things simpler, but you should definitely still know how to do it without one."
