I think this thread will be especially useful this year as it was in 2008.
I have the spreadsheet, and it's useful for approximate calculation, but I'm interested in something a little more exact.
Here's a screenshot of the calculator:
I see down on row 36 they have whited out some text that is labeled "PSI modulus for Primeline NR compound", containing the values 105, 154, 202, 331, 460, 1100, 2800, and 4400. This corresponds to 100-900% elongation - they multiply these constants by the "CSA" value to get the estimated force.
I didn't find anything matching that sequence at
http://oeis.org/.
Looking at a curve fit of that function, it appears most likely to be an exponential function, rather than a power function or polynomial.
Does that make sense?
Also, I was testing a similar material myself (a resistance band), and achieved permanent deformation at only 300% elongation. (In this case, it was only 16lbs weight) It wasn't a huge visible difference, but it worries me:
At what point do you achieve permanent deformation with surgical tubing? Does any elongation whatsoever cause some deformation?