I built an CS-RK3 into a 622 rim with 42mm tires on to. I installed a 11 speed 11-42 cassette because mountain 11 fits on a road 10 speed free hub. I installed a 42 tooth chainring so as to maintain a 1:1 drive ratio. The derailleur was a Shimano mountain M8000/9000 shifted with Road 11 speed shifters and aTanPan to translate road/mountain pull ratios.

The package resulted in a range of 21-146 gear inches & 680% range. I mostly cruised in 3rd/overdrive in the 24-32 tooth cogs at 16-22mph. The hub worked perfectly fine.

I've since moved that wheel set to Mrs. Base2's Salsa Marrakesh and installed 622x32 tires. 20.5-138 gear inches. She doesn't ride terribly fast, but the tall gearing allows her, her preferred 60-70 cadence with out being in the fast wearing 11, 12, 13 end of the cassette.

Hypothetically, on a 20 inch 406, with 50mm tire you'd have 15-102 gear inches with out exceeding 1:1 which is that hubs actual limit any way. A derailleur that is long enough to accommodate a 42 tooth big cog has it's own concerns with ground clearance. 1:1 gets you your 15gi low no matter what chainring & big cog you choose as long as they are equal tooth counts. All you need to do then is decide how big of a chainring nets you the top speed you want and choose ring and cassette range accordingly.

I like the RK3 and am seriously considering building another wheel set so I have another of my very own.

My only complaint, if you can call it that is the freehub engagement is a function of the ratio selected. (Underdrive/direct/overdrive) This is magnified by the cog selected. Meaning if you are coasting in overdrive in a big cog you may need to turn the crank set as much as a ⅓ to ½ revolution to engage the pawls to drive the wheel. This is not really an issue in direct or under drive or towards the small end of the cassette. It's hardly a problem. Just a source of minor irritation, sometimes.

FWIW: I'm used to fast engaging free hubs and I tend towards faster than the utility/recreational cycling end of the spectrum. So, I may be hypersensitive to very wide range cassettes and hub engagement speed. IOW: YMMV.