wouter,

I remember you once stated how few spreader rake you uses. if you don't change it for single handed you definitely will end up in those problems you describe!

I don't agree in the believe diamond tension can save you.
first comes spreader rake, than comes diamond tension.

remember I once put landenbergers sailings tip here into this forum. just read it! if you miss something read it twice... but than try more prebend! you will discover a complete new world!

best might be to change by 30-40 mm to feel really the difference. than you go back to shore and decide if you test again with 15 mm even more or it already was to much and you choose for less... its important to do this changes quickly after each other to sail in the same wave and wind conditions, otherwise you cannot judge it! of course diamond tension should be kept constant. changing diamond tension is fine tuning, spreader rake tunes the essentials!

have fun!