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: Ohlins 30mm frnt cartrige kit / JRi piggyback triple shock / Brembo RCS / galfer superbike lines / pf 07 pads / Leo Vince sbk corsa Ti full exhaust / Stock revalved ohlins damper / Armor Bodies / Servobuddy / Skutr.net captive spacers / Vortex Rearsets / Bazzaz FI/QS/TC/AFM / Motion pro revolver / XT gpx / kit clutch mod / SS built motor