Rock The Bike: Rejoin the chain.
Rock The Bike: Cut a length of trashed innertube to the chain you cut.
Rock The Bike: Drop the chain into the innertube all the way.
Rock The Bike: Break the chain at the length of the loop you measured.
Rock The Bike: Take a trashed chain and measure a loop. Remember to set the seatpost at the high end of the range where you like it.