When I started vaping, I got sample packs from Vapewild and OhmsVapes. So when I started to DIY, I had a dozen 10ml bottles to experiment with my mixes. It can also give you an idea of what flavors you like and you might be surprised by what you find out.
As a smoker for going on 50 years, I only have 3 or 4 taste buds left. So I use the guideline of 20% when I do my flavor calculations (I sound so smart when I say that), but when I make the mix I multiply the flavors by 1.5. This kind of makes the juice flavor boosted and works well for me. That's the great thing about DIY, you can let your imagination run wild. If you use the 10ml bottles to experiment, you can just throw out the bad juices and the recipes and you've not lost much. Do not get discouraged when you get several juices that taste like crap cause it'll happen. It's just part of the process until you finally get the one or two you are looking for.