Arjan,
in case you bought the loos gauge pictured/posted above, 38 is the medium setting for nacra F18. Max is 42. The prebend of your mast must be inline with the cut of your mainsail (luffcurve matches prebend). If you crank the stays hard, the mast gets stiffer in the lower section and the top is more flexble and vice versa. Andrew Landenberger has some tuning tips for A-cat on his homepage. I found it very helpful to better understand the physics behind. Most People overestimate influence of stay's tension compared to correct use of mast-rotation and cunningham.