On a monthly basis how much top-up salt should be added to a saline system swimming pool of approximately 75000 liters?

This would vary on conditions for e.g if the pool overflows because of rain more salt needs to be added the amount of water lost through backwashing is another reason. Evaporation is not a problem as this leaves the salt behind in the pool. a salt tester for the water is required to work out the amount of salt needed. these are available from pool shops. B T W you should not have to add salt to the pool on a monthly basis if you have done the job correctly to begin with.