Looking through TSO-C3e, paragraph 3 states:
3. REQUIREMENTS. New models of turn and slip instruments identified and manufactured on or after the effective date of this TSO must meet the MPS qualification and documentation requirements in SAE International’s Aerospace Standard (AS)8004, Minimum Performance Standard for Turn and Slip Instrument, dated September 1, 1975.
I don't have access to this standard but it should give you a specific lateral acceleration (g) for the slip indication.