Line for preventer?

[memelet]

I've rigged amsteel lines from boom end to near goosneck with shock cord inside to keep them snug. (Shock cord idea came from BrionT. Very clever!)

I will now run the second part line from the bow back to the cockpit. What kind and what size of line should I use. Boat is 42 feet, main is about 400 sf.

Also, what should be the weak link? Don't think it will be the boom lines. The padeyes at boom are quick hefty with big backing plates. 1/4 amsteel will probably outlive the boom braking. (But my boom wall is very thick.) I'm using a colligo soft shackle blocks at the bow, and thats roughly 10k lbs break. So I'm guessing it has to be the long line that should break, no? But again, what kind of line and what size?

thanks much!
-barry

(rigging is just too much damn fun -- really cuts into time under sail ;-)