For cartos,
tanks, clearomizers, etc...
Kurt (ECF chemist) recommends (and so do I, having learned it from him) keeping your flavor + distilled water + PGA + PG = ~22%
PGA thins more than distilled water does, and DW water thins more than PG does, so that influences things some
PGA can tend to brighten up flavors and make them pop a bit more, much like larger quantities of PG
I've never encountered a downside to thinning to be honest. If anything, it opens up flavor, and obviously improves wicking. I've yet to see it negatively impact vapor production. When I drip, I leave things alone. If I'm making something I know will end up in an
rba or similiar system, I use Kurts equation (honestly I ignore the PG component, because I find that it takes loads more PG to thin than PGA, PGA based flavor or DW).
the three vendors recommended above are top notch, some of the most humble and knowledgeable people in these business