Zender... Yes, there is, but you may not like it.
Listen to your cartridge in your system in your room with your ears, at different loads. Find the load that sounds best. Sorry, but ultimately it's the only way.
The experiences of others and perhaps even the manufacturer's specs can get you close. But there are too many variables for the "perfect" load to be calculated. Every electrical component in the chain from cartridge to speaker coil has an effect, and changing the electrical load on the cartridge changes the physical behavior of the cartridge suspension itself. It's just too complex to model.
This is one reason I chose and love the BentAudio Mu stepups. I can achieve virtually any imaginable load in a matter of seconds by swapping in a new resistor. I can even use pairs of resistors to achieve intermediate loads for which no single resistor is available. Easy, educational and cheap!