You want the chain length to give you a good range of adjustment on your motor mount for the gears you might run. Another way is to use a different chain length for every gear change to keep the motor in the same place for best weight distribution but that's really pullin' hairs. I try to work it so I can just use a standard length of chain right out of the package.
I am curious now. Next time I scale I will do it with the engine in a couple of locations just to see how much the numbers change. Usually I run 120 link chains right out of the box.