LoudFastUgly lists a 12.25" eye-to-clevis Redwing with 100/80 lb/inch progressive springs, or a 13.00" eye-to-clevis with 110/90 lb/inch progressive springs.
The spring rate numbers mean that it takes 90 pounds of load per inch of travel for the intial movement until the closely-wound coils "bind" (touch each other), then the rate stiffens to 110 pounds of load per inch of travel thereafter.
Stock shocks on the 400F are typically 12.50 inch long. I would recommend raising the rear 1/2" rather than lowering it 1/4" (unless you are very short-legged). The 110/90 spring should be fine.....I don't think it would be necessary to go with a higher-rate spring.
The thing you need to know is how the "free length" of the spring compares to the installed length on the shock. Assuming the difference is about 1 inch, with a 90 lb/in spring on dual shocks, you have to put 180 lbs of force on the pair of shocks just to get them to start to compress, and an inch of loaded sag from static position (360 lbs total shock loading) wouldn't be unreasonable. Keep in mind that the "sprung weight" of a 400F plus you is maybe 520 lbs, and more than half the weight is over the front wheel. The shocks are subjected to somewhat more force than the rear tire carries due to their position on the swingarm and mount angle, but still I think you can see that 110/90 is a reasonable spring rate.
