With that spec showing overload occurs at 5.7mV output from the cartridge when the phono is set for 64db gain, and given that the cartridge has a nominal output of 1mV, then for a hypothetical 20db of overhead margin, you would like to see at least 10mV before overload. 5.7mV is about 15db of margin above the nominal 1mV of output. "db" are in log scale so half the voltage is not half the db. As others have mentioned, 64db is probably more gain than you need, but the next lower gain you can select, if it is 40db, is too little gain. You are better off with 64db, IMO. As for the rest of it, just listen. If you hear distortion on loud passages, maybe the cause is phono overload but maybe not, as distortion on loud passages can have many other causes as well.
In general, the overload margin of a phono stage is inversely proportional to its gain; the higher the gain, the lower the overload margin, but since you use high gain with cartridges that have a very low output to begin with, overload is less likely to be an issue. Also, in general, tube phono stages tend to have a higher overload tolerance than SS ones.
