I scale my recipes in an effort to achieve the same end product. By that I mean the same OG, FG, IBU's, ABV, color , etc...
For the grain bill I adjust the weight while keeping the same percentages. Hops are scaled by keeping the IBU's the same. Yeast is adjusted by unit volume and OG as per mrmalty.
Water is adjusted in accordance with the final volume desired.
Starting volume = final volume + grain absorption + evaporation (boil-off)