Gill, are you talking about spi sheet or halyard length. At a guess because you mention trapezing you are after the optimum sheet length.
About a year ago Gary (twice bitten/ altered/ Aus formula Blade/ whoever?) posted an excellent photo of a line from the tack of his kite approx 1000mm long with a loop which the downhaul line passed through prior to he first spi patch. So when you doused the kite the spi sheets were pulled into the snuffer very quickly preventing running over them whilst at the same time taking up excessive spi sheets on the tramp. I have always meant to try this but to date have never actually got round to doing it. Maybe somebody will re-post this image as it is quite hard to explain, picture.