I use color coded Die springs from McMaster Carr, I used the red
https://www.mcmaster.com/9584k76(medium/heavy) rate in the rear and blue
https://www.mcmaster.com/9573k73 (lighter) towards the front. I am going experiment with a gold
https://www.mcmaster.com/9588k448 (stiffer) in place of the red on my next run. From the research I've done you want more stiffness towards the rear where it is rigidly mounted and progressively lighter towards the front.
The links give specs on rates, dimensions etc...