Shimano GRX RX 815 Series Dealer's Manual page 120

ADJUSTMENT
Adjusting the rear derailleur
3.
Press shift switch [X] while turning the crank, and move the guide
pulley toward the largest sprocket.
Move it to the position where the chain makes contact with the 4th gear and a subtle
noise is heard.
TECH TIPS
• It can move 16 steps inward and 16 steps outward from the initial position, for a
total of 33 positions.
• In adjustment mode, the guide pulley will overrun slightly and then move back
in an exaggerated manner so that you can check the adjustment direction. When
checking the positions of the guide pulley and the gear, perform the check when
the rear derailleur has come to a stop.
4.
Press shift switch [Y] five times while turning the crank, and move the
guide pulley five steps toward the smallest sprocket.
This position will serve as the target for adjustment.
Shifting switch [X]
Shifting switch [Y]
120