Yes, spring rate is linear, but the point at which the wastegate opens is determine by the spring preload set by the adjustment nut
Here's a diagram of the system with the actuator on the left and the wastegate arms on the right.
The amount of force that the spring puts out is Fs=kx where k is the rate and x is the amount of compression. This is where the linear spring rate lives. The spring force is pushing left on the diagram above. Pushing right to open the wastegate is the force of the pressure inside the actuator Fp=PA where P is the pressure and A is the surface area of the piston.
If Fs is greater than Fp, the wastegate will be closed since Fs is pushing left in the diagram above and holding the flap against the housing. The cracking pressure of the wastegate is when Fs = Fp or kx = PA.
Since k and A are constants we can see that the spring preload (x) affects the cracking pressure (P).
So nubcake, your way of setting the wastegates works because you set the preload of the springs equally between the two sides by turning the nut around twice. I'm going to go the other route of determining the cracking pressure from one side (since I don't know how many turns of the nut they went at the factory) and adjust the preload of the spring on the other side until the cracking pressures are equal.
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