Mixing can be tricky especially since every flavor is different. It would be nice if they were all the same % and so on...at least in my perfect world it would be
Here are a few tricks. Start with low. You can always add more flavoring. Adding too much can cause your liquid to taste chemically or have almost no taste at all. They type of atty you are using can also impact this. You'll find if you are using a dripper you will need far less flavoring but with a basic
tank like a CE5 or Pro Tank you will need considerably more flavor. It is key to note that the hotter the coil burns the stronger the flavor is going to be and oftentimes liquids can taste different on different devices.
I would recommend starting at 3% for strong citrus flavors. Make a batch without the nicotine (if you are using any) and adjust the flavoring at a 0mg. When the flavor is where you would like you can then use this as your base. Take a bottle and add the amount of nicotine you need to get your desired strength. Then fill the rest of your bottle with the base you made. This will keep your nicotine strength constant.
PG based flavors are stronger and tend to carry much better then VG based flavors, however, plain VG will make e-liquids sweeter. Try mixing your liquid at a 70/30 starting with 3-5% flavoring and go up up from there.
I sincerely hope this helps...Let me know if you have any other questions!