Depends on the size of the headsail you'll be polling out. Our boat has a 13'8" 'J'. That pole length is too short for optimally poling out the genoa
for optimum sail shape. Have a 15'6" 'penalty' spinnaker
pole which is also to short. The telescoping 12'-22' spinaker extended out to about 19' seemed to be about right. Unfortunately it couldn't take the load and pretzeled the 2nd night of running wing and wing DDW in 12-14 mph winds. The shorter pole winged the sail out well but because of the short length didn't flatten the sail for optimum DDW sail configuration. The sail still set okay but was better with a longer pole.
Don't know of any formula for the pole but the length should be less than the length of the foot of the sail that it will be used with by a little. Personally, wouldn't spend the money
for a dedicated twin headsail down wind
rig. Have spent little time far enough off the wind
with a sail poled out. Got lucky on the sail to Hawaii
and ended up DDW most of the way but normally that sail would be a reach. Wing and wing worked just fine for that sail.