Short as possible without having the chain under loads of tension. For the derailleur, try removing the top pully and backside of the cage, then reverse the bolt for the lower pully and use a bolt to secure it. Use the limiter screws if possible to lock the derailleur in the right position, or if needed, a short section of cable and housing. Without the top pully, the knuckle will be held against the dropout by the spring tension, so it won't knock, and the single pully wheel has less drag then the chain twisting between the two pullies. The other neat feature of this mod is that because the cage is now one sided, is is easily disconnected to allow for easy wheel removal. I'll try to see if I can get pics of this set up.