I have found this process pretty easy for me, so I don't have to handle the nicotine each time and it is easier for me to measure at larger numbers.
I get 60mg Nic and combine it with PG/VG to get it down around 12MG. I leave 10% of PG out, for flavoring wiggle room.
For example, I do a 50PG/50VG mix and store in a 60ML bottle. I will then add:
30ml VG
12ml PG
12ml PG with Nicotine
That makes 54ML and leaves me 10% room for flavoring which is PG based.
I will then usually make juices in 10 ML increments, which means I will measure 9ML from my base bottle and then add flavorings around 10%. If I add more flavoring than 10%, it will cut my nicotine below 12MG but I'm not that particular as long as I'm around 12-6MG