Total volume of diluted alcohol = (Starting alcohol % / desired alcohol %) x volume of starting alcohol
So if we wanted to dilute 6 L (1.6 US Gal) of 90% ABV alcohol to 40% ABV, then:
Total volume of diluted alcohol = (90/40) x 6 = 13.5 L
Therefore, you would have to add 7.5 L (2 US Gal) of water to the 6 L (1.6 US Gal) of 90% alcohol which would ultimately give 13.5 L (3.6 US Gal) of 40% ABV alcohol.
Please note: That the formula above is only an approximation and adjustments will have to be made to get it spot on at 40% ABV. The Still Spirits alcoholmeter can be used to determine the ABV of the spirit.