Originally Posted by
smd4
Here's how you determine chain length for my DA 7700 derailleur: With the chain on the smallest cog and the largest chainring, the two pulleys on the rear derailleur should be perfectly vertical--90 degrees perpendicular to the ground. Straight from the manual.
Works every time for me.
Having both RD pulleys "perfectly vertical" also depends on using a cassette that is officially within specifications and the B screw adjustment.