Phono preamp or SUT for low output moving coil

to be a bit more precise, the SUT is a Step Up Transformer so the ratio is 1:X not X:1. That said it is indeed as Lewn stated. Take the square of the turns ratio and divide the input impedance of the phono stage to calculate what the load is on the cartridge. So a 1:10 is 10 x 10 = 100 and the typical 47KΩ divided by 100 = 470Ω

However, the main reason you have an SUT is to get enough gain, it should not be viewed as a means to achieve a desired load resistance. Much better to have a phono stage with a very input impedance like the EMIA LR (300kΩ is standard), select an SUT that gives you the desired voltage gain, and then load the primary of the SUT to get the desired load resistance.

Despite what others may tell you... SUTs are a superb way to get wonderful sound if you know how to implement them

Another point is the unfounded obsession with a perfect RIAA response. The idea that the RIAA has to be perfect is ridiculous given all of the other variables in frequency response in any system including the room. Why obsess with .01dB of accuracy with the RIAA when other factors give deviations in response many  orders of magnitude greater? All else equal then yes it should be as good as you can make it, BUT nothing else is ever equal so relax and go with the music. Go with what sounds the best, not measures the best. Don't chase specifications to ridiculous precision.

"Much better to have a phono stage with a very ______ input impedance like the EMIA LR (300kΩ is standard),"

Herman, the word you omitted in describing the input impedance of the EMIA is “high”.

thanks, yes, I did unintentionally omit the word "high" , but "very high" would be an even better description 

The EMIA has a high input Z to facilitate using it with LOMC cartridges that work best into high-ish impedance,

yes, and to be more specific, if you have a fairly standard 1:20 SUT into 47KΩ the highest load you can achieve is 118Ω (47,000 divided by 400) .. Into 300KΩ that will be 750Ω. If you then want it lower you can load the primary. Slagle has a handy calculator on his site. https://intactaudio.com/tran.html and scroll down

the standard 47KΩ came about back in the days of MM only cartridges and is not ideal for those of us who use MC. Here are a few interesting reads, All credit for what I know about it goes to Dave Slagle.

https://www.stereophile.com/content/dispelling-myth-about-phono-cartridge-loading

https://intactaudio.com/SUT%20design.html

Actually, you can change the load resistor in any MM stage to whatever value you prefer, if you can identify it and can solder.

I wouldn’t go so far as to say "any" since the input impedance may be determined by more than a single resistor, but it is certainly the most common way. Hopefully whoever is in there replacing  resistors knows enough to determine what is needed.

The caveat with going too far is that increasing the input impedance also increases the amount of noise that gets in, so there is a practical limit