qclabrat, The size of the loop to the RD depends on where the cable enters the RD and where the frame's housing stop is located. There isn't really a set length. I use a long enough length of housing that a flush fit into the RD is accomplished regardless of the angle of the RD when on either bottom or top cog.

Loop size grew as more cogs were added to the rear because the cable moved less for each single cog shift and friction from a too tight of a radius caused imprecise indexed shifting.

Brad