If i'm making up a bucket of grout to pour over a concrete base ready for screeding, i'd get around three quarters of a bucket of clean water, and stir in enough neat portland cement, to give me a bucketful of RUNNY, POURABLE grout. If you make it too thick, you'll never get it out of the bucket. Another way i've done it, is to soak an area of floor, and then throw handfuls of dry cement onto the wet floor and brush it around in with water. So really, there's no specific ratio as such,, just wet the floor, mix up the grout, and pour it over an area of floor to screed on. If you leave any grout in the bucket, it'll thicken up, and be of no use, so just mix enough for a workable area.
Roughcaster.