Calculating

[kubota]

Can anyone give me a formula to do this:
Say I had 1litre of alc at 95% and i want to reduce it down to 40% , how much water do I add in?
Or if it was 1litre at any given percentage and I needed to reduce it lower, what is the amount of water to add?
What I'm trying to get away from is adding water and testing the % and then allowing for the temperature difference, and then adding more water and so on.
There must be a formula , that once ya know the percentage of alcohol and the given amount, 1litre is easy to work with , you could just dilute it with water and it would be at whatever percentage you want.