Less than maneuvering speed (calculated for your weight); greater than stall speed (calculated for your weight). Both IAS. BUT, your ASI may have significant errors due to the pitot and static ports being mis-aligned wrt the relative wind. Since not everyone uses the same ports, the only way to know for sure is to test at a safe altitude.
since you probably want to slip to increase your ROD on finals, you’ll already have some flaps out, therefore < Vfe of 87KIAS.
For what it’s worth, a full flap slip using about 70KIAS on my 3 bladed CSP bird results in a healthy descent