It depends on the size of your spinnaker, mainly the luff. Remember there is no standard sail for the Hobie 18SE
When I bought a hobie 18 spinnaker from smyth I put the tang 2.5 feet above the hounds per Randy's reccomendation. I also ran with about 2 ft of pole prebend. I'm not sure what the luff was on the spin though.
I ran it of a comp tip for a while no problems until I found a solid aluminum mast.
I'd just make sure you get a tang with at least 4 rivets and rivet it well. I'm not sure the newer style halyard bails would work as the mast track isn't too strong. I just had a regular tang with block off it.