If you want to know how much water to add to dilute your alcohol
down, just multiply the amount of spirits you have by (strong/weak) - 1
eg: if you want to dilute 2L of 75% alcohol down to 40%, you
will need to add 2 x ((75/40)-1) = 1.75L of water
If you want to know how much alcohol to use to make a known quantity,
multiply the final amount by (weak/strong)
eg: to make 1.125L of 40% alcohol using spirit at 75%, you will need
to use 1.125 x (40/75) = 0.6L of the 75% spirit, then top it up to 1.125L
using water.