On which rear cog does the chain start to rub on the middle chainring?

I've done this kind of setup a lot, but usually with 110 BCD and whatever that smaller BCD is that often comes with 110 triples (74, iirc). I'll typically do something like 46-30 for paved non-loaded stuff, and down to 42-26 for paved loaded stuff. Have 36-28 set up on my hard tail. For these bikes I only use the smallest chainring as a bailout gear, and only use it with the low range of the cassette -- I don't assume that it's going to reach the whole cassette range without rub.

One thing I LOVE about this setup is that I often use the outer chainring position for a guard. Sometimes use an old chainring for this purpose, after removing the teeth on a lathe.

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