Originally Posted by Andrew R Stewart

Maybe the missing info is that the OP has friction shifters... Bet not though.

The compatibility aspect aside the first thing I thought of was that the OP did two changes which will make pulley knock more likely. Both a larger rear cog (and that it's about at the der's limit of size, why do we push the limits of der capacity and then question the results) and the smaller ring will make the upper pulley ride closer to the large cog's underside.

When sizing a chain and if the der capacity hasn't been exceeded I place the chain on the small/small combo and see how long the chain can be and not have any chain/cage contact or droop. then I add a link. Cut the chain here and trial fit on the big/big. Then see how many links can be removed before the cage is too far wound out. My final cut is somewhere between these two lengths. Yes it's a judgment thing. But I would rather have one more link then minimal then run at the shortest length limit. Andy.