Google is partially correct. Some recipes does come out at 15% yes.
HOWEVER! Different recipes has different percentages, therefor you cannot count all recipes has to be 15%. Some are below, some are above. I have seen many great recipes that runs at around 4%.
So please do not use whatever advice you have found on google about percentages.
It also depends what concentrates you're using. An example. Many TFA and Capella concentrates. Yeah 15% total in a recipe is normal. But if you are using say real flavours and wonder flavours who are super concentrated. Than a recipe made up of those concentrates, only 5% can be too much.
It really depends on a lot of things, the flavoring, what you mix it with, manufacturers, your own taste and I would even go as far as to say it depends on where you're from. Seriously I've seen US stores/manufacturers and DIY vets recommend a minimum of 15% for the exact same flavorings that EU stores/manufacturers and DIY vets recommend at a maximum of 5-6%. Best to start with single flavor mixes at a recommended % and go up or down depending on your preference, try some simple recipes found online and try to adjust them to your liking.
Certain flavor can be 0.25% and go up to 8% or more. I tried making my own at different percentages and it wasn't bad but everything tasted a bit bland or bitter like dark chocolate. I took some recipes from a website that showcased a few good ones and just copied those. I tried to make my own and now I just copied thai coconut icecream, goofy juice and boss reserve clone for now to compare.