@ains :
based on your question ...
i made myself a chart of wind strength with sail size mapped against board with fin:
so, in 20 kph winds (~11 knots) i will use JP SLW92 with 10 m² sail and 66 cm Select slalom or 58 cm True Ames SB weed fin
and i put litres/width as a board indicator as well
you can also see that sails in 8.0-8.5 m² range can have fins from 30 cm to 58 depending on the board !!