We owned a B30 Cruiser before the B40 Cruiser. I actually made up a whisker pole for her, but cannot find the pole measurements I used.
Failing being able to find the "J" measurement, it can be measured on the boat. The "J" measurement is the distance from the mast forward and horizontal to the furthers point of the vessel. Often this is the anchor roller or the front of the pulpit. If you make the pole longer as Jake suggests using the "J" measurement of the jib, the pole is just that bit longer and harder to stow.