Depends on length of hanger and size or ring up front. 1st attempt I managed to get an SS derailleur to 36t with B-limit pushed to max. Other time it barely worked with a 32t.
Needed the Roadlink as was running a 46t up front. Frikking love di2. Plug. Play. Experiment.
Easy to do that, just get a roadlink and any 6870 derailleur. 11-40 M8000 cassette. sorted.