What length spinnaker?

[KnuckleDragger]

I would like to add an asym cruising spinnaker to my Hunter 38 for sailing down the Pacific Coast and cruising in the Sea of Cortez.
I have my eye on a couple of used sails that look appropriate but am not sure of the overall length needed. If I were buying new I'd just let the sailmaker figure it out.
The boat is a fractional rig with an "I" dimension of 46'8". The spinnaker halyard is ~1' above the forestay/jib.
Since the foot of a spinnaker will fly above the pulpit that seems to indicate a shorter length, but the "belly" of the sail when flying indicates some additional lenth.
Need some voices of experience here...

[daddle]

The luff of a spinnaker should match the hoist from tack to head. The tack being wherever you desire...usually about lifeline height in a cruising situation. You want to be able to make the luff tight. A little shorter is way better than a little too long. The hoist height of a sail is measured by attaching a tape measure to the halyard, hoisting it all the way, and measuring to the desired tack position.