Our reply: Paul, you'll notice the recommended range is 3mm to 6mm. Why? Changes to front alter the geometry at a much different rate than changes to the rear. Meaning: there is no 1:1 ratio.
For the 2003 WR250F you need a heavier spring even WITHOUT a YamaLink. The stock spring is set for a rider about 160 pounds. At your weight you'll need a heavier spring, also, with a YamaLink to achieve proper sag and balanced handling.
The stock spring rate is a 4.8kg. At your weight WITHOUT a YamaLink you need a 5.2kg or closest to achieve proper sag. WITH a YamaLink you need a 5.6kg spring to achieve proper sag.