I note that American's frequently use the # sign. Can you explain what it means?
What would you use to measure the 0.485" (say) in the 55' shroud to ensure that you don't tension it more than 15% of breaking strength?
# for American "Engineering Notation" is Pounds. It can be either LBF or LBM; it's just a shrot-hand for Pounds because it's called the "pound" symbol.
To measure the 15% tension I'd just use a standard Loos Gauge. Although the calculation predicts 0.485" extension I would not bet my life on it. Young's Modulus is a theoretical constant for the material and it is calculated based on lab results under ideal conditions (perfect sample, solid section, etc.). If the theoretical is within 20% of the actual tension in LBF I'd call it a good comparison; but that's not as close as you would get with a Loos Gauge.
If you wanted to measure the extension I would put a pair of calipers on the open body turnbuckle and measure the distance between the threaded ends. That's a direct measure of the amount of elongation you are putting into the shroud.