How much water do I add to cut 2 L of 63% spirit down to 40%?

The quick estimate uses conservation of alcohol: the ethanol volume before and after dilution is the same, only the water changes.

Alcohol present: 2.00 L × 0.63 = 1.26 L of pure ethanol.
Final volume needed: 1.26 ÷ 0.40 = 3.15 L.
Water to add: 3.15 − 2.00 = 1.15 L.

Two refinements matter in practice. First, ethanol and water contract on mixing, typically 1 to 3%, so the true water requirement is slightly more than the simple arithmetic suggests; the calculator’s OIML Precise mode handles this. Second, add the water in stages and re-measure with an alcoholmeter at 20°C between additions, because overshooting cannot be undone without stronger spirit.