The suggestions above are for the shortest chain possible. Many derailleurs wok better with some more chain that that.
Campagnolo recommends the longest chain possible, measured with the chain threaded through the RD and on he small sprocket and pulled up the RD takes up th slack in the lower loop. Shimano recommends something in between.
I prefer running chains to the long side, and so use the Campagnolo method, but sometimes take an extra inch or two off that.
In any case, ALWAYS double check the shortest length method, because accidentally cutting a chian too short can destroy a drive train.
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