You are ready to add sorbate when the wine is done fermenting and the sg is stable for 3 days in a row, but add sulfite first as sorbate added with too low a sulfite level will result in a geranium smell and flavor. You will need to add 1/2 tsp per gallon after adding the 1 campden tablet per gallon.
doesn't the geranium smell and taste come from adding sorbate to a wine that has an active malolactic fermenation going?