A 2008 WR250F rider asked about rear suspension spring rates and lowering their Yamaha WRF. The weight was listed as 65kg.....
"With the 2007-2011 WRF Yamaha lowering link's extra 10% leverage, the bike and rider will achieve much better plushness, traction and soak up the square edge and braking. To achieve this the sag must be rest and the bike properly sprung. The stock 2008 WR250F spring rate is 4.7kg/mm.
At 65kg with the stock setup you need a 4.93kg/mm or closest.
Then we add 10% to the 65kg weight and input approximately a 72kg rider weight and get 5.09 or closest....and closest is 5.2 kg/mm which is great if the rider carries a pack or tools or water. And it allows them a bit of range on the sag setting."