The real issue you have is not the pole length but the distance from the tack to the clew. You also need to make sure you have enough to pull the leech tight enough. If you know how high the clew is off the deck when fully hoisted and you determined where you want the spin sheet line lead you could extrapolate the length of the pole.

Here is what you could do:
Hoist the chute in zero wind.
Give the clue to your buddy.
Determine where you would like to place the block for the sheet to the deck and hold your sheet line there.
Have the clew moved forward and back until the sheet line angle is 45 degrees (equal pulling back and down) to the sheeting point.
Measure distance from front crossbar to the tack point.
Keep in mind that the spinnaker pole should have a fair amount of prebend so you do not break it.

Will this work for you?
Dan DeLave