Can't make sense of that, in the context of the recipe. One tablespoon is about 15 ml, so maybe a 'scant' tablespoon is about 10ml? Why not just say "160ml water" instead of "150ml and a scant tablespoon"?
Batters can be a bit finicky; and accurate proportions of ingredients matter up to a point, but not that accurate! I don't know about fish batters, but I'd imagine it's got to be of a consistency that you can dip the fish in the batter, and it sticks to it. If it's so thin, it runs straight off the fish back into the bowl, no good. If it's so thick, you can only spread it on the fish with a plasterer's trowel - no good. Experiment!