Question about loading with an SUT

As others have well pointed out, you should never stress about differences like 18x versus 20x. It truly doesn’t matter. If you swap from one brand/model of SUT to another, the inherent sonic differnces between those SUTs - due to materials choices, core size, lamination quality, wire windings, etc - will absolutley swamp out anything caused by small differences to loading and/or step-up ratios.

When playing with SUTs, what you need to get a sense of is the "bounds" you’re working in:

• If you load a cartidge down too heavily, you’ll start to lose increasingly more than 1dB of signal level, due to your phono stage’s input being on the "wrong side" of a voltage divider. This is a problem when you use too high of a SUT ration for a cartridge based on its coils’ DC impedance. You would’t want to use a 40x SUT with 38 ohm coils, beacause you will be losing 6dB of signal! This can also cause frequency response aberrations, besides the bulk signal loss.
• You can calculate out the losses, and you probably want to stay lower than 2dB loss in all cases. You’re fine with what you’re using here - well under 1dB of losses.The "load your cartridge at 10x its coil impedance" isn’t a bad rough guide. You can always go higher, no problem. Going lower, I find 5x - 6x to be the cutoff - lower than this is almost always bad.
• If you choose too high a SUT ratio - even if the loading works out OK and the signal losses are acceptable - you run the risk of encroaching overload margins on your phono stage. At first this can cause the sound to seem "dynamic, punchy" but as you push it the sound can become harsh, gritty. As a rule you need to keep the signal hitting your phono stage input under 10mV as calculated by the cartrdge’s rated output level. E.g. a Shelter rated at 0.5mV with a 20x SUT = 10mV (it will actually be a little less due to loading losses described above). It’s generally best to target 5mV, but up to 7.5mV is fine.
• Avoid cartridges with "inefficient" magnetic generators like Benz Ruby / Gullwing / LPS (Ebony / Wood / Glider series is fine), Dynavector Karat 17D, there are a few others. These cartridges have high coil DC resistances paired with relatively low output levels - they are NOT a good match to SUTs or "transimpedance" head-amps. The ruby plate armature (as opposed to iron / permendur) is at fault here. Ortofon has started using some non-magnetic armature materials in its top cartidges, but thy reign it in (hybrid) just enough to still allow use with carefully chosen SUT ratios.
• "Transimpedance" head-amps are the active analogue to a SUT (passive) - sort of. The loading (as close to 0 ohms as possible) is really weird and totally different. You can buy a Hagerman Piccolo Zero for cheap ($270), and it’s a lot of fun. Its musicality competes with much more expensive SUTs, at the expense of a higher noise floor. It’s almost silly how well this device matches with certain Ortofon SPUs (Classic GM Mk II spherical; Meister Silver) - this has actually been my favorite combo of the past couple months.

You left out “head amps” aka “pre-preamps” which have been around since the early 70s. These devices are active gain stages that boost cartridge output voltage ahead of the phono stage. They do not correct for RIAA. Sutherland and Hagerman make good ones but there are many other choices. When MC cartridges hit the US market in the 70s, most audiophiles owned only MM phono stages with insufficient gain to accommodate the MCs. Pre-preamps were the cure for that, even before there were many SUT choices. The good news is that a prepreamp will work with any MC, regardless of its internal resistance. So there are 3 viable options.

@chipcalzone , I'm using a Declare Z3 phonostage.

Thanks for all your responses. @atmasphere , this procedure is beyond my capabilities and leads me to consider a head amp.  In fact, many of the responses seem to be leading me in that direction.  I'm also good with the "don't worry about it" camp since I'm quite happy with the sound I'm getting.  Perhaps a Hagerman Piccolo is in my future but I think from this discussion, I'm going to rule out a different SUT since it seems they need to be tailored very specifically to the cart/tonearm cable and I don't want to have a different SUT for every cart.

A high gain phono stage can be an MM stage with an additional voltage gain stage tacked on to the input end. Such a topology can therefore handle both low and high output cartridges. Alternatively a high gain phono may simply incorporate a very high gain but unitary input voltage gain circuit.

I use five different carts and use one SUT currently wired at 1:10.