I tried posting this on the San Juan 21 forum
and didn't get any takers so I'm trying it here. Keep in mind this is a very small boat and the pole in question weighs about 5 pounds.
I got a telescoping whisker pole from the PO when I purchased my boat, broken so that it no longer telescopes. I haven't measured, but I think it is technically a bit too long for the working jib and a bit too short for the genny. All the same, I've used it with both sails on several occasions and it definitely helped some. What I did was just clip one end to the small loop of rope dangling from the clew of either sail, I assume for that purpose, and the other to the padeye on the mast, then hauled in the windward sheet until it was somewhat taut.
I've just read some other info on using a whisker pole and there was talk of using a topping lift and foreguy to set the shape properly. Does anyone find that to be necessary or helpful on small boats?