Go Long
I recently installed a new Harken furler on my 1978, HC33. I took some sage advice and replaced the forestay also, went up one size larger. I measured the forestay longer than necessary, assembled (dry fit)everything on the ground and then hoisted and connected the top stay fitting to the mast. I walked the furler (had a friend help) to the bow and eyeballed the entire unit before the final cut of the new forestay. This may not be the best way to do it, but it worked for me, and eliminated the worry of ending up too short. Also, allow extra room for tensioning of the unit. I did end up tightening the back stay some. One other suggestion...if you can talk to a rigger in your area, they may have some inside professional tricks they would be willing to share for a DYI guy. It might just be worth your $$ to hire the rigger and let them assume the responsibility. Good luck.