Here are some ASTM International Standards for water quality. Max conductivity... just meet the standards.
Ultra-pure water - https://www.astm.org/Standards/D5127.htm
Deionized water - https://www.astm.org/DATABASE.CART/HISTORICAL/D1193-99E1.htm
ASTM D1125 Standard Test Methods for Electrical Conductivity and Resistivity of Water - https://www.astm.org/Standards/D1125.htm
ASTM D5391 Standard Test Method for Electrical Conductivity and Resistivity of a Flowing High Purity Water Sample - https://www.astm.org/Standards/D5391.htm