Unless your base liquid is very close to what your target strength is you'll need to do the math first. for example, if your liquid is 36 mg and you want 18 mg you need to cut it 50/50 to get 18mg. But it wont be 50% flavoring it will be 50% VG/PG/Flavor and 50% base. I personally would play with the flavor mix to get it close before adding the nic base, then make minor adjustments because the base will change the flavor and TH slightly.