Welcome Creature
You've gotten some answers, but I'm going to throw some more info your way for consideration....
1. Your end goal is to cut 24 mg to 12 mg. Short and simple answer is to add 5mL of "something" zero nic (flavoring, more PG and/or VG)
2. Do you want to maintain the 25PG/75VG ratio? (I'll do some math below)
3. If you're "less concerned" with PG/VG ratio, you'll need some plain PG and/or VG in addition to flavor concentrates (which are often PG-based) to "top-off" after you mix a flavor.
Figure you're probably going to add somewhere between 5-20% flavoring (depending on what you're mixing), which in a 10mL bottle is anywhere from 0.5mL-2.0mL of mostly PG. On the low-end of flavoring, you'll need about 4.5mL of unflavored "something" (PG/VG - NO nic) and 5mL of your 24mg nic solution. To maintain the 25P/75V ratio with 5% PG flavoring, you're looking at 0.75mL plain PG and 3.75mL plain VG.
For 20% flavoring, you won't be able to maintain the 25P/75V ratio. Math just won't work.