As you approach 30 ohms from higher values, you may notice at some point that you have less apparent phono stage gain, due to loss of signal from hot to ground. If that is happening, then go up at least one notch from the value that affected gain. This will give you a near approximation of "critical damping". Or just use 100 ohms, which is pretty standard for cartridges with an internal resistance of <10 ohms.