Lou the rule of thumb for spinnaker poles was for the length to be equal to the J measurement (Tack fitting to mast base). The correct length for the whisker pole will depend on what sail is being poled out so an adjustable pole will be of most use.
It should be the length of the foot of the sail that you are using. That is why whisker poles are adjustable. If you got your 110 up in then its that length. If you are going with the 155 then you make it longer.