I need a new spinn sheet for my N570, it's go an F18 spin and the setup is very similar to that of an F18. My current sheet is about 40foot and too short for trapping. What do you length guys run?

It varies from boat to boat (spinnaker cuts and sheeting points) and height of crew.

Rig it up in light winds with a thin piece of over-length cheap rope that you don't mind trimming down to get the right length

also depends if you will use a pigtail, blocks on the clew, etc.