It's a total nic and total juice thing.
If you have 100 mg/ml unflavored nic base, it'll bump pretty fast. For instance, you want 18 mg/ml final concentration, using X ml of your 12mg juice and Y ml of 100mgml unflavored.
X*12mg/ml + Y*100mg/ml = (X+Y)*18 mg/ml
For 10ml of initial 12mg strength juice:
10*12 +Y*100 = (10+Y)*18
120 + 100Y = 180 + 18Y
82Y = 60
Y = 60/82, about 3/4 of an ml
Quick check:
10*12mg/ml + 3/4 * 100 = 120 + 75 = 195 mg of nic
195 mg/(10 + 3/4) = 195/10.75 = 18.13 mg/ml
So, if you have 100 mg/ml unflavored nic juice, about 3/4 of an ml of it added per 10ml of your 12 mg/ml juice will bump it to 18 mg/ml final strength.
This is a pretty minimal dilution.
On the other hand, say you just had 48 mg/ml unflavored nic juice. Again, how much is needed to bump 10 ml ("X") of 12 mg/ml base juice?
X*12mg/ml + Y*48mg/ml = (X+Y)*18 mg/ml
120 + 48Y = 180 + 18Y
30Y = 60
Y = 2 ml of unflavored 48 mg/ml nic juice added to your 10 ml of juice to get a final 18 mg/ml concentration. This is a bit heavier dilution.
Quick check: 10*12 + 2*48 = 120 + 96 = 216 mg of nic.
216 mg/(10 + 2) = 216/12 = 18 mg/ml
(100 mg/ml is pretty strong stuff -- exercise precautions if you work with it, eye protection recommended if you're spazzy like me, wipe up any spills promptly, don't leave it on your skin, keep it and any waste materials safely away from children and pets.)