I normally go with about 25 litres of water which i bring to strike temperature, about 68c. I then add my grain bill, usually around 5kilo's or various malts. Leave to mash for an hour. I put a colander on top of a seperate clean bucket and lift the grain bag onto this and let the extra liquid drop out into the seperate bucket. Whilst this is happening i turn the temperature up and get the water left in the boil pot up to temperature to save a few minutes. I get around 3 litres of water at 70c or so and pour this gently onto the grain back through tin foil with holes in to sparge. I then put all the sugary wort from the bucket into the boilpot with the rest and bring it to the boil and crack on with the hop additions. After an hour boil im usually left with around 19/20 litres ready to ferment at about 1050 O.G.
Dont think my efficiency is that great but its quite a stress free method so i dont mind having use slightly more grains.