Propylene glycol is not a common solvent that people measure solubility of pharmaceuticals in. e.g. you'll not find PG solubility figures for basically anything in the Merck index. So if you cannot find it on the internet, you must measure it yourself experimentally. There is no easy equation to predict solubility accurately: some algorithms can give you a binary "yes, no" decision but they are functionallys useless for determining exact maximum solubility.
The maximum solubility of benzos in PG will also depend on the ambient temperature and whether or not the PG has water in it and if so how much. Warmer=more dissolves. More water mixed in = less dissolves.
