How often should a "relief charge" nurse be assigned as charge nurse? Head charge nurse has charge pay rolled into her base rate whether she is charge or not. Relief charge only paid differential while assigned as charge. Fair?

OP here: For context I am assigned as charge when primary charge nurse is working. She is getting charge pay while being staff nurse. I do not receive charge pay unless I am charge. It is currently 60/40 split of how often I am charge. But next month I will be charge more than her.

Base rate when she is not in charge. Rate of pay when assigned as charge should be that which the head makes. They're doing the same job, should be same pay.