I propose you use the software "buffer maker" of ChemBuddy. In case you dont have this software, send me the concentrations of NaCl and phosphate types for calculation and next send to you the answer. In my case, weight compounds and mixes in same recipient, next add water, disolve and diluted to end volume. Always the pH is in value specified. In my case the sterilization is by filtration, using 0,2 um filters. If PBS is do not sterilized the max storage time is around one week. Regards