Hi all, had some time to profile the NOS springs, can't do old springs yet as I have lent my hydro pump out. I'll do the old ones in the next month or so as I'm going to change both of the front suspension arms (my NOS L/H arm arrived from Canada today!).
I've created a simple test rig out of a few things lying around in my workshop/garage. The only things I bought was the two pulleys (don't buy Zenith brand ones, as they are crap) and a digital weighing scale that can measure up to 45kg from Super Cheap Auto for ~$11.
Here is some pictures of the setup:


I created a simple MS Excel spreadsheet to calculate both the
initial tension and the
spring rate.
To measure spring, I stretched the spring 1" and took a reading in pounds, and then again at the 2" mark.
This is the excel spreadsheet:

Here is a
spring calculator that I entered the spring measurements into. I guessed the material as hard drawn... don't know if correct, but didn't make much difference:




Conclusion:
I think my NOS spring has not lost any significant initial tension, and very closely matches the values from the spring calculator. It will be very interesting to see what the old springs measure, when I get them of the car. My methodology is a bit crude, mainly because the Zenith awning pulleys have poor bearings (or lack thereof) and using a digital weighing scale has readability issues as the value jumps around as well as me not providing an even tension.
Let us know what you think, and if anyone want to drop off or send me an old tired spring, or a copy of the Excel file just PM me.
Cheers. Jim