Thanks Wonky Donky for the information. I think my measure of preload equals yours ... I generally assess with the spring in its free state in the cylinder. Preload is then how much the spring has to compress to get the trigger block or retaining block etc back in place.
With the HW rifles, a short length of spring at the cylinder end means you can engage the block threads with ease... joy!
With regard to 1/8 wire and 3.2mm. No problems but I note that their are many rifles needing a wire less than 3.2mm ... some 3mm and I know from experience that the .2mm can make a difference. Some original springs are 2.8mm (think HW55 and some early prewar Dianas) Also, a 3mm wire spring is often wound to a smaller od than a 3.2mm one such that a 3.2mm one does not fit. Iguess that the smaller od of the 3mm wire makes it as stiff as the larger od 3.2mm one? I have never tested this.

Regarding original sizes ... when I put my mainspring database together, I took a lot of sizes from Cobham's book. He lists 3.25mm for the Mk3 ... so 1/8".

Cheers, Phil