I have a single spreader rig. I use 1000# in the lowers and 800# in the uppers. The slack in the leeward shrouds depends on how much tension I have on the backstay, how much mainsheet I have on, and the number of people on the rail. I think it is not a good measure of anything.
I have about 1000# in the uppers and just over #500 in the lowers on my single spreader rig. What have you found if anything that is a benefit to having the uppers lower than the lowers? I do not have the jock strap or the beam on my boat so tensions can very.
It’s just the way I’ve always done it. At some point a professional rigger pointed out that the lowers on larger boats are generally of a larger diameter than the uppers and that is because the lowers must work against the force placed on the spreader by the cap shrouds. So…the lowers need to be tighter in order to keep the lower part of the mast where it belongs. i.e. there is a lot of force pushing the mast to leeward at that point. The jock strap and BOD will form another set of spreaders below decks and will help to get the load off of your vintage flexi-flyer. I think you should make your move. It does make a big difference in rig tension. You should keep in mind that the jock strap will be offsetting the load of BOTH the cap shrouds and the lowers, so it should be a larger diameter rod than either. The point is that you don’t want it to stretch, which is to say, allow the boat to flex. Properly done, the loads are enormous.
That’s my two cents on the subject.