Strawberry-kiwi sounds nice. The good news on that one is many berries don't need a ton of steeping (not sure about kiwi) but it may be a shorter steep.
With a mix generally unless it's a known shake and vape flavor, I'll steep for at least 4 days or so before I even think of trying it and now I'm on a mixing schedule, I'll wait at least 10 days but most of my mixes steep for far longer. Unless you have a flavor that fades (lemons, sometimes mints) it's usually fine to steep longer. If you test right away, sometimes you'll get something you think is good, and it may or may not be, but often flavor will seem to vanish for the next week, and you haven't done anything wrong if so, it's just the flavors starting to blend with all the other ingredients, so if that happens, put it aside for a bit.
You can use things to "decrease" steep times (it may not even be necessary with your strawberry kiwi mix, like a magnetic stirrer, heat, or an ultrasonic cleaner. I've occasionally done a "quick and dirty" heat steep if mixing for others, but I just use Very Hot tap water and sit the mix in there for 30 minutes to an hour to get things going.
I personally prefer time, the occasional shake, and etc. It's just more consistent for me that way. Some rapid steep methods can mute some flavors.
And yes, learning to use a juice calculator is pretty essential. There are many good ones so you might want to check a few out and see if you feel "confident" in your math (it's kind of like algebra working backwards, you put in your target mix and then work backwards to make sure you are adding what you need to). Best of luck,
Anna
