You will probably not be able to select that combination in E-Tube Project, but that's OK.

When configuring synchronized shift you basically tell the bike when to shift from big to small ring (or other way around) by telling it at what gear # (cassette) the shift should happen.

For example, in the image below (on the left) the shift from Top/50T to Low/34T happens at the 25T - the second cassette sprocket.
Even if you were to install a different cassette, the gear shift would still happen at the position. Di2 itself does not really care about the number of teeth all that much.

So when using a different/mismatched crankset or cassette you generally select the closest match available and then ignore the number of teeth / gear ratios and just look at the shift 'positions' to set up your synchro shfit.



(this is an E-Tube Project 3.4 screenshot and I tend to use it to show the full-synchro limitations of FD-6870 / FD-9070, so ignore all the extra information on the image ;-))

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Perfect, thanks.