the chain length is set in webcontrol menu Settings->Advanced Settings
This number is to be a mutliple of 6.35. When you do the chain extension in the setup (watch the videos on quick calibration and chain extension I made here) it will know where it is. The home is then automatically set at the center of the work piece if your measurements are accurate. the initial extension will be high on the board unless you change the chain exension lengths to match your center.