I posted one here a few months back.

However, the run salt and silver are always the same molar concentration.

A general rule is that if you want to go from an SR emulsion to an RS emulsion, you remove from the kettle the amount of salt equivalent to silver. The remainder stays in the kettle with the gelatin. Then, the salt removed from the kettle is made up to the same molar concentration as the silver and they are run into the salt + gelatin at the same molar rate.

This is a starting point, not a formula.