Ideally by driving the car and observing what it does in corners. That’s tough to do, so trial and error is next. Upping your spring rate both front to rear stiffens the car up, but wont change the balance front to rear, which is what you want to do. If I was to guess, I would imagine you’d want to raise your rear spring rate and leave the fronts alone.
Does this sound right?: You’re looking for as much front grip as possible and less rear grip, but in a controllable package. It’s not like taken a car with equal front and rear grip and just beating the hell out of it to get the rear to stay out while you mash the pedal to keep the smoke coming. High speed drifts are easiest then in cars with MORE front grip, not LESS rear grip. In other words, if we’re making changes, ideally we’d want to add front grip, not just reduce rear grip. Either one gives you that front/rear balance, but gaining front grip is a better way to do it.
I’ve never understood how suspension tuning companies pick the spring rates in their coilover packages because I’ve never seen or heard of one getting it right. Luckily the bigger companies use commonly sized springs so you can just pick your rate, height, and diameter and order them for $60 anywhere. I have 800, 850, 900, 950, and 1000lb rear springs for my car sitting in a box in the trailer. 
Luckily once you figure this one out, you wont be changing them often or ever again.