Thanks, wfr42 - if that thing's correct and my poor Maths is accurate, I'm wayyyy under-carbonating.
At 18c (my usual bottling temp), it recommends 253g per 23L. That comes out at around 5.7g per bottle. That's just over 1tsp, so in my past batches I'm about half-carbonating (at 1/2 tsp per...