I would recommend starting at 10 percent. If the flow is adequate, you are good to go. If not, bump it up a bit, but 15 percent is about the maximum before it gets too thin, IMO. Good luck!
The only problem with too thick is slow wicking may cause a dry hit, which can ruin a cotton wick . Draw slowly until you are sure the liquid is wicking well.
Water is much more effective at thinning VG than PG is.
Are you going flavorless? Is there any PG in your flavorings? How thin/thick are the flavorings?
Start low. Really low.
I have made PG-free liquid using as low as 3% DI water.
You should be able to have it thin enough to wick well at below 10%.
you may want to try 10% vodka to dilute thats 10% of complete mix i myself burn the alcohol out of the vodka before mixing i find it adds to throat hit maybe my imagination thou anyone else agree/disagree
Put simply, vodka is 40% ethanol and 60% water. If you are cooking off the alcohol, you're just adding water (along with trace amounts of starch, etc. but those are negligible). If you are getting an increased throat hit, it's because there is still alcohol present.