I think I have found a scientific document on this very topic written by a guy with a PHD
Only thing is it needs translating into something ordinary people understand.
------------------
This site does a good job of explaining in plain English how the stretch factor works and how it is measured when the manufacturer gives it to you, yes there a standard way of doing it so all figures are from the same hymn book.
MountainZ.co.nz - All about mountains. Mountaneering, rock climbing, ice climbing, skiing, snow boarding.
-------------------------
Most rope manufacturers will provided % stretch usually.
In this link
New England Ropes - Climbing Products you can click on the ropes and get some figures.
In this link you can also do the same but these are dynamic ropes and stretch factors will be given.
New England Ropes - Climbing Products
Other figures around the net show that dynamic ropes will stretch as much as 50% before breaking, where as our static ropes only 15% ... but that's to breaking point which we dont do. This can be seen around page 13 of the attached document.
MountainZ.co.nz - All about mountains. Mountaneering, rock climbing, ice climbing, skiing, snow boarding.