The amount of shunt current depends on your build. Your CCS should always be slightly more than the maximum amount of current used.

Look at it like this: you set the current the PSU supplies. What isn't used, is transformed into heat. So you do not want to set it too high, as you would raise the temperature of all components thereby shortening their lifespan. On the other hand: if you set it too low, there will be drops in voltage. With a setting of 50mA, you're good for most stock builds based on the TPA modules. I can't give you better guidance without exact measurements of what your specific setup's power usage is.

Please read the specific manual for your version of the PSU. I do not know what PSU version you are using, but mostly shunt is the voltage measured TP-OUT / TP-SHUNT.