The whisky is diluted as part of the manufacturing process to the optimum(ish) level: 80 proof / 40% ABV. You are correct to not dilute it further.
Ref: Whisky is distilled to around 70% alcohol by volume (vol-%) then diluted to about 40 vol-%, and often drunk after further slight dilution to enhance its taste.
Ref: Whisky is distilled to around 70% alcohol by volume (vol-%) then diluted to about 40 vol-%, and often drunk after further slight dilution to enhance its taste.