Been there, done that on my 28 before it got sold.
Yes, the books recommend the pole to be the foot-length of the sail.
The boat's J measure was 11'10" or 3.6 m long and the 12'6" - 3.85m /60mm/3mm long spinnaker pole was not really long enough for the genoa with a foot of 16'4" or 5.0m.
I had a 16'10" or 5.15m/60mm/3mm long whisker pole for my gennaker with a foot of 22' or 6.6m. That set up was working just fine. I used the same ends as on the picture above.
The length of 6m/60/3mm tube was about $65 from a tube place in Auburn.
I recommend to use as heavy a tube for the whisker pole as one's spinnaker pole because there are huge loads developing on the pole under certain conditions and a pissy thin pole might not handle the loads. I was warned by the rigger about complacency!
It was not heavy, l think it was .65 kg/m, easy to handle even in high winds and gave plenty of opportunity to work out the right combination. The spinnaker up-haul was used to lift the pole and maneuver it around.
When goose winged two jibs, l just used an extra up-haul for the second pole while scandalized or dropped the main.
It worked a treat every time.