The theoretical calculation is by determining the center of buoyancy,
the center mass of ballast and calculating the righting forces that
would react at the top of the mast ... when the mast is pulled over to
a heel angle of 45 degrees.
You can do the same experimentally by restraining the boat (so that it
can roll unimpeded) and by pulling horizontally (or calculating the
trigonometric difference for other angles) from the top of the mast
until the mast is at a 45 degree angle to the horizontal ... then
MEASURE the stress/strain in the wire (cap shrouds). The induced/
forced 45 degree heel would develope the maximum static loads in the
wire (by the magic of trigonometry).

This would establish the max. 'actual' target load in the wires ....
and (important) then make everything 4 times as strong for 'offshore',
3 times as strong for 'coastal' and twice as string for 'inshore'. I
prefer a higher Factor of Safety (FS=6 ... many of the reknown
'offshore' designers tout FS=4 but when you backcalculate their work I
find FS 'well above' FS=4) for offshore.

Simple answer .... your designer probably knew what he/she was doing
in rigging selection by adding the proper safety factors after
calculating the max. theoretical shroud loads ......... and typically
(on a purely mathematical basis) the cap shrouds would be tensioned
for a 12-15% load (mast/boat not heeled) based on the ultimate tensile
strength of the exisiting wire, ditto forestay/backstay and all the
other shrouds would then be trigonmetrically calculated based on the
12-15% of the caps and backstay. Very simple answer .....12-15% on
all shrouds, then go sailing to see if the mast stays perfectly in
column while on a heel approaching 45 deg.

Intermediates .... probably next to worthless from a stress
standpoint. Do the trig. calcs. and you'll see that they react with
extreme tension to the chainplates because of their very shallow
intercept angle with the mast (hounds) .... use runners instead of
intermediates for better mathematical solution and less stress/strain
as developed on the intermediate chainplates.

;-)