When I find a flavoring "mix" I like, I also tend to mix up a 30ML batch of JUST flavoring.
Then use some small bottles to mix up different strengths, just a matter of adding your pre-mixed flavoring to the unflavored base, then letting them steep about a week.
DIY tip of the day #2 - Write EVERYTHING down and label your mixes. I had a mix I got impatient on once, didn't let it steep enough, tried some variants, etc.. Tried the same one I hated about 1.5 weeks later, loved it, and had absolutely no clue what the mix was...
So, yeah, whether its on paper, in Excel, in a DIY calculator, doesn't matter, just write it all down and keep notes.
Trust me, nothing is worse than getting something just right and not being able to quickly recreate it...
Thats a good starting point. Lately I have been mixing 17%-18% with my strong flavor note taking up about 12%.
Also, starting with fruity flavors is uaully easiest, at least fruits are the easiest for me to make that is.