The bottom line is that it's up to you.
Maybe it would be useful to remember that what we're talking about here is a highly simplified version of how the real ship was rigged. The plastic deadeye assembly represents two deadeyes (round pieces of wood, each with a groove around its circumference and three holes drilled through it), secured to each other by a lanyard that passes through the holes. In actual practice the backstay is seized around the upper deadeye. The backstay runs up to the masthead; there it either is seized around the masthead or passes around it and back down the the corresponding deadeye on the other side (or maybe, depending on the rig of the individual ship, to the next deadeye aft on the same side of the ship).
If I were doing it (assuming I hadn't decided to ditch the plastic deadeye assemblies in favor of aftermarket parts), I'd probably mount the deadeye assemblies - firmly - to the hull, secure one end of the backstay to the deadeye assembly, take it up to the masthead, seize it there, and take the line down to the corresponding deadeye on the other side. In any case, when putting the finishing touches on the rigging, be sure the masts are lined up right. Just a little unevenness in tension can yank a mast out of alignment quite a bit.
Hope that helps a little. Good luck
Youth, talent, hard work, and enthusiasm are no match for old age and treachery.