IMO it's a bad idea to be messing with 100mg base every time you want to mix up 10ml of juice. So this is what I do.
I use pint canning jars, and mix 50ml of 100mg (in PG) with 150ml of PG and 200ml of VG to get 400ml of 50/50 mix @ 12mg to work with. I then put that bottle of 100mg base in the locked deep freeze.
Flavorings need to be in a water soluble base (no oils). I use flavor concentrates, and only add 1 drop of flavoring/2ml of e-liquid which drops the nicotine down to about 10.5mg. (15 drops in a 30ml bottle of e-liquid)