Easy with an accurate steel measuring tape. This method will work for ALL sizes of wire and regardless of the wire diameter. The 'target' tension % for most sloops is 15%. The following is a fairly precise method that only needs a 'steel tape ruler' some plastic tape and your brain.
Unload the tension (slack off) then measure and accurately place 2 marks on each shroud at precisely 79.0" (2.0m) apart.
5% tension = .04" (1mm) stretch .... (per 79")
10% tension = .08 (2mm) stretch .... (per 79")
15% tension = .12" (3mm) stretch = approx. 1/8" ..... (per 79")
Cant be any simpler and you dont need a rigging gage only a steel tape measure.
15% is the 'benchmark' tension for most rigging on sloops.
Measure/Mark all the shrouds, etc. when 'unloaded' (~0% tension)
Bring up the tension in 5% increments (ie. all shrouds AT 5% tension), then ALL at 10% tension, then ALL at 15% .... so that the final 'stretch' results in the 79.0" distance ... stretched to 79.12" = approx. 79-1/8" for 15% tension.
Works on ALL stainless steel 'wire' rigging and no matter what the diameter.
Once you have 15%, take the boat out in ~15kts. and while beating (heeled), put your eyeball next to the mast slug track and 'look up'. Then make final corrections so that the mast is 'dead straight' while sailing in ~15kts. of wind when on a beat.
Other.
The above does not include 'prebending' a mast. Masts with backstays and with a single set of spreaders should have 3/4" forward bow somewhere near the 'middle' of the mast; if multiple spreaders then 1/2" forward bow per spreader set - 1" forward bow if mast has 2 sets of spreaders, etc.
Method: once rig is at 15% tension, loosen aft lower shrouds (usually attach to the mast AT the spreaders) and tighten forward lowers until you 'guestimate' the 3/4" forward 'bow', then retighten aft lowers, etc. until you get the ~3/4" 'forward bow' in the mast. Prebending will radically increase the stiffness of the mast and will prevent 'mast pumping' ... plus will tend to 'flatten' the mainsail a wee bit (your sailmaker already cut the sail for this expected prebend, so if you dont prebend the mast to match the sail you will have a bit more maximum draft than the design - not a big deal if you are only 'cruising'.). Prebending compensates for the amount of bend that is imparted to the mast from the sail when the wind is at approx. 15-18kts --- and the force from the wind-loaded sail on the mast tends to put a 'reverse bow' into the mast (which 'powers-up' the sail). If pre-bending is confusing ... simply ignore this.
Hope this helps.