When you're experimenting, you don't worry about your exact PG/VG ratio or nicotine level.
You start with XYZ, taste, add flavorings as needed, taste, etc. etc. IF you get something right, you go over your notes and figure out how much flavoring you have in there and make a new batch with your PG/VG and nic levels where you want them and see if the new batch works. To keep waste to a minimum, most start small and work up.
If you have a pre-mixed
juice, and are adding to it, you do much the same thing. Take a small volume of the premixed out in another bottle and start experimenting and keeping notes. IF it works, then you have a record of what it took to get it there and you can make a mix of that, in the right ratio, and mix a larger batch of that recipe with your premixed. (Unless you are diluting the flavoring of the premixed, there is no way to maintain PG/VG, etc. You're probably using PG based flavoring and increasing your PG as you add flavoring.)