Also, consider the type of flavor you'll be using, because the percentage needed can vary quite a bit.
tobacco flavors like FlavourArt require only 1-3% flavoring, but some fruit flavorings are weak and may need as much as 25% to suit your taste.
A large percentage of flavoring will reduce the nicotine strength and also thin the liquid if the flavorings have a water or alcohol base.
This calculator works well for simple recipes:
eJuice Recipe Calculator
Using the calculator, these inputs:
Nicotine Base: 50mg/ml
Target Nicotine Level: 10mg/ml
Amount to Create: 6ml
Water Added (optional): 0%
Flavor Percent: 10%
Yield these results:
PG or VG with nicotine: 1.2ml 24 drops 20%
PG or VG (no nicotine): 4.2ml 84 drops 70%
Water: 0.0ml 0 drops 0.0%
Flavoring(s): 0.6ml 12 drops 10.0%
Since you want 20% VG in the final mix, just multiply 6 (mls of total mix) x .2 to get the amount of VG needed: 1.2ml.
Subtract this from the "PG or VG (no nicotine) row above; this leaves 3.0ml, which will be plain PG.
Your recipe would look like this:
PG or VG with nicotine: 1.2ml 24 drops 20%
PG (no nicotine): 3.0ml 60 drops 50%
VG (no nicotine): 1.2ml 24 drops 20%
Flavoring(s): 0.6ml 12 drops 10.0%
I recommend using a 1cc syringe to get accurate measurements.