A difference of 4 cm in the tip and tail wouldn't make much difference apart from maybe more ollie height. Try to keep the rail between your feet to start out and boardslide it. The boat (or ski, cable, whatever is towing you) will pull you off the rail naturally since it won't be in line, unless the cable you're riding does line up directly overhead.
Forgot to add - bend your knees a lot, more than you think is necessary probably and try to stay off your edges when sliding, keep the board flat or you will slip out.
(Message edited by jtnz on December 22, 2009)