i imagine it changes all the time. depends on the correlation which sometimes is very high between the two and at other times they seem to have a separate life of their own. the higher the correlation the easier it is to calculate. probably an easy way is to find the correlation, then tic match the number of bunds X the correlation %. trouble of course is that cor is always historic over 10 days (usually) and if it falls over you will only find out as it happens. too many tentative variables for me but good luck. i'm sure someone can give you a better idea how to calculate it.
