It depends on what you want from the
vape -- good strong flavor and excellent
throat hit, or tons of vapor?
PG provides stronger flavor, supports the nicotine
throat hit much better; high PG can even compensate for lower nic, for good throat hit.
VG provides thicker vapor and more of it; if you don't want such a strong throat hit, then lower PG, higher VG would be a good choice.
Many people start with 50/50, and depending on how it strikes them, go more towards one or the other. If you have any type of breathing problem, VG might make it feel/seem/actually be worse; VG is a lot thicker, and causes a thicker coating in the airways as the vapor condenses. But many people find PG to be quite irritating.
50/50 is probably a good place to start, for many people. I started at 70%PG/30%VG, because I knew I wanted a strong throat hit, and then I found that I like 85% PG even better.
Andria