The best day you'll ever have is to spend it at a track with a suspension tuner.
If this is too expensive, ask around to see who does good work (you might find someone local who specialises in mini bikes), give them your weight fully kitted and a description of how you ride. That will get you ball park. Don't exaggerate your ability though, you'll end up with a setup way too stiif and hate your bike.
98 CR125 01 CR250 99YZF R1
Thanks to: Motorex / D.I.D. / Vesrah / YES Helmets