Freddofrog said..
I'm ~ 75kg.
Foil/wing/boom/wetsuit/helmet etc ~ 10kg
Common belief says you need your weight in litres, ie 75L, but shouldn't it be 85L, or am I missing something?
(assuming no wind)

No, you are right. However, in practice:
- Some things underwater also add a bit of volume: the foil, the pad.
- Often the board volumes are the ones gives by the shaping software. The added PVC sandwich layer may add a bit of volume
- Salt water add a bit of floatation. About 3%, nearly 2,5 "virtual litres" to a 75L board.
- And most importantly in practice, you will sink if perfectly still. But the slightest speed (wind push, slight paddling) will add an upwards push from the board and foil. This is the main factor explaining why you seem to float on a board that should theoretically sink.