You can get a good estimate of the answer as follows.
1- for every inch you move the wheel back you'll need 2 inches of chain (4 links )
2- to see whether the chain is long enough, shift into the big/big combination, the pull the lower loop forward by that number of links. if the RD still has room, you're OK.
This doesn't mean you'll have the optimal length chain, just that it's long enough to work.
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