Here's a simple formula for making a specific quantity at whatever nic strength you want.
Easy peasy formula for any nic base strength, target nic, and qty desired:
Step 1: (Total quantity desired X target nic in mg/ml) / (nic base strength in mg/ml) = amt of nic base required
Step 2: Amount of zero nic liquid to add = Total qty desired - amt of base required
Example: I want to make 150ml of 15mg/ml unflavored. I have 24mg/ml base and plain VG.
150 x 15 / 24 = 93.75ml of nic base
150 - 93.75 = 56.25ml of zero nic liquid (VG, PG, etc.)
ETA: Your solution will be exactly 20mg/ml:
144x20/24=120ml of nic base
144 - 120 = 24ml of PG