This is a tough one to answer with the amount of info here.
First, I'd recommend doing an internet refresher on math, as I think that you really should know math like this before working with 100mg/ml nic liquid. I've had to do that a couple of times in the past, and math was my favorite subject in school. [This was just a sensible disclaimer, and not meant as any sort of slight, as I don't know you, but would not want to think of any harm befalling any of those who might read this.]
That aside, it really depends most upon what your target nic concentration is. With only 20% allowed for VG, you would not be able to get anything higher than 20mg/ml with 100mg/ml VG base, since 20% of 100mg/ml plus 80% of 0mg/ml would equal 20mg/ml. For 20mg/ml, the rest would be plain PG and flavorings (considered PG,) in this case. If less than 20mg/ml, then you'd have to add a bit of plain VG to make up the difference.
If you went with a PG nic base, then you'd have no effective limit on nic concentration, and would need to add the proper amounts of plain PG plus flavorings, with 20% plain VG. For instance, if 20mg/ml were the target, then you'd mix 20% PG nic base, 20% plain VG, and 60% plain PG and flavorings.
So, with 80% PG and 20% VG ratio, you have more flexibility if you choose a PG base. If your target nic concentration happens to be 20mg/ml, though, then choosing a VG base could be slightly simpler.