Eibach used to do progressive rate springs for cars (have a front set for a chevette somewhere) which have varying coil spacing, the theory being the closer coils go coil bound and effectively increase the rate. to look at (dead technical i know
) the upper 3 or 4 coils are equidistant, the next few increase slightly in their respective coil 'gaps' and then the rest ( the majority) are reasonably well spaced as you expect for a saloon car spring. this is supposed to give a more progressive effect than the dual rate achieved with spring and tender method. the springs i have are 'tarmac' spec so am guesing in region of +/- 700lb/in dependant on whether they were designed to be softer initially, increasing to the rated rate, or vice versa (never fitted them).
not hugely technical i know, but might help in your quest, they may even do them for Pugs still
CNH