Careful... I did some basic calcs on this and yes; for an equivalent diameter and equivalent length the linear stretch is roughly proportional to tensile stress regardless of diameter. BUT if the smaller diameter shroud is smaller in length (which it should be) then the amount of pre-load you apply per mm of stretch is increased. That's because strain = change in length / original length.
Let's say you have a 50' mast with 7/16 upper shrouds. The calculations I did suggest that you would need 5.98 mm of stretch applied to them to get ~1600 lbs of pre-load.
In the same example; if the shrouds were 1/4" you would also need to stretch them 5.98mm to achieve 540 lbs of pre-load.
But; if you have a shorter mast (as you should) with 1/4" wire the stretch required to get 540lbs of pre-load will be less. If the mast is 30' tall the stretch required will be 3.6mm to get the same 540 lbs of pre-load. If it were tightened to 5.98mm it would have roughly 900 lbs of pre-load or 25% of break load.
So; while the rule-of-thumb is probably OK for a rough tune I would say that if you apply it to taller rigs it will result in shroud tensions that are on the loose side; and on shorter rigs it will result in shroud tensions that are a bit too tight.
Thanks for the additional info on rake VS bend. I understand it now; and now I'm not sure if the mast actually has rake or not. I'll do some measuring and adjusting according to your excellent procedures.
KH..I don't know how you did your calculations, but I confess you are puzzling and confusing me a lot...
where did you get the numbers and are you sure about the values you are using for breaking loads?
Normally 7/16 cable which is around 11mm has a BL of 27.815 lbs, roughly (as my tables are metric)..and a stretch of 5.98mm as you suggest is 30% not 6%, as that is the stretch for the 1600lbs you are refering to.
a 1/4 cable that has a BL of 7054 lbs roughly, at 900lbs it is at 13%...