I find it depends on the airflow in your
device (at least for me it does).
On my old GGT with no cartomiser holes, I get leakage with just PG, so I have to top up with VG - probably to no more than 10% although I don't measure it - I do it by the feel of the atty now and bung in a bit more as needed - I am not very exact about it at all.
On the GGTS however, with its extra airflow, I tend not to need (or want) VG as the liquid tends to flow into the AFS slightly less.
So, what I am saying is, trial and error is your best bet - and it will probably change with every atomiser and with atomiser age too as the AFS hole gets more broken in.