You cut the new chain to length on the bike after you install the new sprockets. get the axle into the middle of the adjustment range and wrap the chain around the sprockets. Mark the link you need to cut.
Make sure you cut the proper link and not an inner link out so you can actually install the masterlink.
__________________ Mod list: GP 25mm front cartridge kit / JRi piggyback triple shock / attack rear link / Brembo RCS / Galfer SBK lines / Leo Vince Factory Corsa R full exhaust/ BMC Race Filter / Yosh Case Savers / Vortex Clip ons / Vortex Rear sets / Vortex Keyless Gas Cap / Lighttech Chain adjusters /AIM solo DL data logger / motion pro throttle / KIT ecu & Harness / Hotbodies WSBK bodywork /