Well Cloughie, it all depends on what type of grass.
Southern rye grass will respond very well to the computer designed Rockgrabber 675/b terrain interface nodules (studs in old money) but extensive simulation tests suggests that Northern snod-grass may well cause terminal friction failure leading to cranium-terrain impact episodes in the less wary. so you might want to consider the Rockgrabber 675/a if you are racing north of the Tees/Exe line.
Those in the Long Mynd area may do best with 675/a on the right foot and 675/b on the left.
Currently the manufacturer recommends two sizes up for 675/a but 1.5 sizes down for 675/b unless you are Scottish when all bets are off.