Your cartridge has a nominal output of 0.5mV. If you are going to use a SUT, then you will be connecting it to a phono stage that has gain sufficient for only a Moving Magnet (MM) cartridge. The canonical MM cartridge puts out about 5mV under the same conditions that yours makes 0.5mV. So to drive an MM phono stage comfortably, you would want to use a SUT that adds 10X gain or has a turns ratio of 1:10. SUTs are not magic; they increase voltage while decreasing current by a like amount, like any other transformer. In other words, the product of voltage X current on the input of a SUT is always equal to the product of voltage X current on the output side. Ergo, the current on the output side of the SUT will be reduced by 10X compared to the input current. But you don’t care, because your MM phono stage is driven by voltage. The other factor to worry about is impedance. A rule of thumb is the input impedance of the phono stage should be about 10X the internal resistance of your cartridge. By convention, nearly every MM phono stage presents an impedance of 47K ohms. The SUT reduces the impedance seen by the cartridge by a factor equal to the square of the turns ratio. In this case, since the turns ratio is 10, the impedance seen by the cartridge on the other side of the SUT will be 47K/100, or 470 ohms. This is fine for any MC cartridge, nearly all of which have an internal resistance <<47 ohms. (47 is 1/10 of 470, thus obeying the 1:10 impedance rule.)
If you are going to use your cartridge to drive a high gain phono stage, also known as an MC phono, you only have to think of the voltage needed to drive your linestage OR the input of your amplifier, in the case where your phono stage serves as a linestage. (A typical MC phono stage will present a load that is compatible with your MC cartridge, but you should always check the values to be sure that the MC phono input impedance is at least close to 10X higher than the cartridge internal R.) Nearly all amplifiers can be driven to full output by a signal voltage of 1V to 2V. You can quickly calculate based on the fact that 60db of total gain is equivalent to a 1000X gain in voltage. So your 0.5mV cartridge output will be amplified to 0.5V by an MC phono/preamp that makes a total of 60db gain. For a comfortable match you would like a bit more than 60db gain. 66db will give you 1V output. (There are calculators on line where you can quickly convert voltage gain to db and vice-versa.) The math is one thing, but in practice there is a lot of flexibility based on additional signal boost supplied by a linestage downstream from the phono stage (which can easily add 10-20db gain), input sensitivity of your amplifier, efficiency of your speakers, etc.
