2.7L per kg is the default setting for most Grainfather systems.
However, 2.4-3L per kg is normal for most systems. You are aiming for a porridge-like consistency of the mash. To work out the mash thickness the following formula can be used:
(mash water volume - dead space volume (G30 is 3.5L) ) / grain weight = mash thickness.
Going thicker than 2.4 or going thinner than 3 can result in a loss of mash efficiency meaning you will end up with less alcohol in the final beer.