Also done this way...
Hello,
Instead of measuring from the masthead, we simply make a pencil mark, as high as we can easily reach, on the front center of the mast. We measure down to the chainplates from there. Essentially we are measuring the hypoteneuse of a very short triangle, instead of a very tall one. At least as accurate. Several advantages: there is no worry about elasticity; we find out where the lower part of the mast is; and it's easier and faster by far.
Centering the bottom of the mast by measuring means we can sight our way up straight.
We also measure out horizontally to the upper shrouds. This will tell us if the mast is situated midway between the chainplates. If it isn't we can compensate accordingly when tuning.
As Ian pointed out, there might also be asymmetries in chainplate height. There are ways to check for this, even when in the water, but I've found that careful sighting from forward of the boat will usually reveal if the chainplates are significantly non-level. Sighting can also reveal if the whole mast is straight, but not coming out of the boat at right angles. I suspect that is what is happening on your boat
As for the measurements, if the tune is good you can indeed use the old wires as templates. The most constructional stretch you'll see is on the longest wires. In your case it will amount to less than an eighth of an inch.
Fair leads,
Brion Toss
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