New different size mast how to measure for new standing rigging

[grboregon]

I guess I must be a dummy but how does one go about it when the mast is down, no standing rigging available to measure. I have re-rigged before when I was replacing and you had an old wire to measure to build a new one. I have no old rigging to go from so the lengths are all different than before. I have searched and to no avail. Suggetions

 

[Charlie Jones s/v Tehani]

I connected the upper end of the wires, then stood the mast up using rope to hold it in place, and fitted the lower ends. All with Stalocs

 

[Will Gilmore]

Do you have measurements from turnbuckle to step height? You can get a very close estimate by laying a straight 2×4 on edge across the step such that the ends overhang the chainplates. Measure both verticle sides equally without bending the board and that's your height. It shouldn't be hard to get the rest of the measurements directly.
Pick out your turnbuckles and they will give you a tolerance that should be pretty generous. Don't forget the Pythagorean Theorem from sophomore high school.

-Will (Dragonfly)

 

[jssailem]

One word.
Trigonometry.

It will get you very close. If you choose to take this path, you might find the Stayloc or HiMod deck end fittings to the shrouds easier to adjust the length of the rigging.